How do you measure for a replacement window?

Key facts

• Andersen's measuring guide (maker), width: "Round down to the nearest 1/8 inch, and measure from three different spots: near the top, at the center, and near the bottom."

  Source: Andersen Windows, How to measure windows, read 24 October 2026.

• Andersen's measuring guide (maker), square test: "If the measurements are within 1/8 inch of each other, your windows are square."

  Source: Andersen Windows, How to measure windows, read 24 October 2026.

• Andersen's measuring guide (maker), insert windows: "The window must also have a minimum jamb depth (space in between inside and outside stops) of 3 ¼ inches."

  Source: Andersen Windows, How to measure windows, read 24 October 2026.

• Marvin's measuring guide for starting a quote (maker), height: "Round up to the nearest inch and record your measurement."

  Source: Marvin, How to Measure for Replacement Windows and Doors, read 24 October 2026.

• EPA (national): "If your home was built before 1978, it may have lead-based paint; the older your home, the more likely it has lead-based paint."

  Source: EPA, Lead Renovation, Repair and Painting Program: Do-It-Yourselfers, read 24 October 2026.

• EPA (national): "Approximately three-quarters of the homes in the United States built before 1978 still contain some lead-based paint."

  Source: EPA, Lead Renovation, Repair and Painting Program, read 24 October 2026.

• Federal lead rule 40 CFR 745.83 (national), minor repair: "Minor repair and maintenance activities are activities, including minor heating, ventilation or air conditioning work, electrical work, and plumbing, that disrupt 6 square feet or less of painted surface per room for interior activities or 20 square feet or less of painted surface for exterior activities where none of the work practices prohibited or restricted by § 745.85(a)(3) are used and where the work does not involve window replacement or demolition of painted surface areas."

  Source: eCFR, 40 CFR Part 745 Subpart E (Residential Property Renovation), read 24 October 2026.

• Federal lead rule 40 CFR 745.84 (national), pamphlet timing: "No more than 60 days before beginning renovation activities in any residential dwelling unit of target housing, the firm performing the renovation must:"

  Source: eCFR, 40 CFR Part 745 Subpart E (Residential Property Renovation), read 24 October 2026.

Do you measure a replacement window from inside or outside?

From inside. Both makers' guides start indoors and measure between the jambs. Andersen says to take the width from inside your home, jamb to jamb, or drywall to drywall if there is no trim, and to take the height from inside as well, head to sill. Marvin likewise says to measure from the inside of your home, from the inner edge of one side jamb to the other, and to measure between the drywall on each side when the window is finished with drywall instead of jambs. Andersen's guide defines the parts: the jamb is the pieces forming the top and vertical sides of the frame, the head is the horizontal piece forming the top, and the sill is the horizontal piece forming the bottom.

Source: Andersen Windows, How to measure windows, read 24 October 2026.
Source: Marvin, How to Measure for Replacement Windows and Doors, read 24 October 2026.

Andersen's guide How to measure windows, read 24 October 2026: "Take this measurement from inside your home. Measure from jamb to jamb or drywall to drywall if you don’t have trim around your windows."

Source: Andersen Windows, How to measure windows, read 24 October 2026.

Marvin's guide How to Measure for Replacement Windows and Doors, read 24 October 2026: "From the inside of your home, use a tape measure, and measure the distance from the inner edge of one side jamb to the other."

Source: Marvin, How to Measure for Replacement Windows and Doors, read 24 October 2026.

Should you measure width or height first?

Width first, then height, is the order both makers' guides use, so write each pair in that order every time. Marvin's guide lists two steps, measure the width and then measure the height. Andersen adds a square test before the width and puts the height third. Andersen's guide takes each dimension in three different spots: for width, near the top, at the center and near the bottom; for height, left, center and right. Andersen says to use the smallest of the three as the dimension. Write all three readings down as well as the smallest, so anyone reading your notes sees how much the opening varies.

Source: Marvin, How to Measure for Replacement Windows and Doors, read 24 October 2026.
Source: Andersen Windows, How to measure windows, read 24 October 2026.

Andersen's guide, read 24 October 2026, on height: "Measure to the nearest 1/8 inch in three different spots: left, center, and right" and "Use the smallest measurement as your height dimension."

Source: Andersen Windows, How to measure windows, read 24 October 2026.

Should you round window measurements up or down?

It depends on which maker's guide you follow, because the two give different instructions. For the width, Andersen says to round down to the nearest 1/8 inch and to use the smallest of three readings; for the height, it says to measure to the nearest 1/8 inch. Marvin's guide is written for starting a quote: it says all you need to start the process of getting a quote for window and door replacement is a tape measure, a helper and its tracking form, and, in its height step, it says to round up to the nearest inch and record your measurement. Marvin also says a window and door expert will confirm all measurements before ordering new Marvin windows. Record which rule you used next to each number, and when you send measurements with a request, say whether they were rounded up to the inch or down to the 1/8 inch.

Source: Andersen Windows, How to measure windows, read 24 October 2026.
Source: Marvin, How to Measure for Replacement Windows and Doors, read 24 October 2026.

Marvin's guide, read 24 October 2026: "Round up to the nearest inch and record your measurement."

Source: Marvin, How to Measure for Replacement Windows and Doors, read 24 October 2026.

How can you tell if a window opening is square?

Measure the two diagonals and compare them. Andersen describes taking both from inside, corner to opposite corner where the head or sill meets the side jamb, each to the nearest 1/8 inch, and says the window is square if the two are within 1/8 inch of each other. Square matters to the kind of replacement Andersen says may fit. Andersen says that if your windows are square, you might be able to use an insert replacement window, which fits into an existing window frame. If they are out of square, it says you will need to consider sizing adjustments when ordering new windows, that repairs to the rough opening might be needed as well, and that for this job you will want an experienced window contractor.

Source: Andersen Windows, How to measure windows, read 24 October 2026.

Andersen's guide, read 24 October 2026: "Start in the upper left-hand corner where the head meets the side jamb and measure to the lower right-hand corner where the side jamb meets the sill. Measure to the nearest 1/8 inch." It adds: "If the measurements are within 1/8 inch of each other, your windows are square."

Source: Andersen Windows, How to measure windows, read 24 October 2026.

Will an insert window fit, or does the whole frame come out?

Andersen gives conditions for each kind. Its insert page says your old windows can be replaced with Andersen insert windows if there is no rot in the window frame, the frame is square, and you want a new window of the same size in the existing frame without disrupting the trim. It says insert windows preserve the original frame, exterior trim, exterior siding and interior casing. Its measuring guide adds a minimum jamb depth, the space between the inside and outside stops, of 3 ¼ inches. Andersen's full-frame page says the entire window frame and sash are removed, and says full-frame replacement is a good option if there is rot in the trim, casing or sill, the window is not square, or you would like to change the size or shape of your window.

Source: Andersen Windows, Insert Window Replacement Installation, read 24 October 2026.
Source: Andersen Windows, How to measure windows, read 24 October 2026.
Source: Andersen Windows, Full-Frame Window Replacement, read 24 October 2026.

Andersen's measuring guide, read 24 October 2026: "The window must also have a minimum jamb depth (space in between inside and outside stops) of 3 ¼ inches."

Source: Andersen Windows, How to measure windows, read 24 October 2026.

Andersen's full-frame replacement page, read the same day, lists among its reasons: "There is rot in the window frame: trim, casing, or sill." and "You would like to change the size or shape of your window."

Source: Andersen Windows, Full-Frame Window Replacement, read 24 October 2026.

Can you measure for replacement windows yourself, or should you hire?

Measure every window yourself even if you plan to hire, and keep your notes next to each bid you receive so the sizes can be compared line by line. Treat the steps below as the homeowner's share of the job, and hand the rest over when one of the hire lines applies. This is practical reasoning, not legal advice and not a finding from any agency.

A do-it-yourself installer needs more than these numbers. Andersen's guide says: "If you’re interested in replacing windows yourself, you’ll need to take further measurements and make further assessments beyond what’s described here."

Source: Andersen Windows, How to measure windows, read 24 October 2026.

1. Gather a tape measure, a pen, paper and, for big windows, a second person. Andersen says: "Before you start, you’ll want to gather a tape measure, pen, and paper. It can be useful to have a helper too, especially if you have large windows."

   Source: Andersen Windows, How to measure windows, read 24 October 2026.

2. Test for square with the two diagonals, each to the nearest 1/8 inch; Andersen counts the window square when the two are within 1/8 inch of each other.

   Source: Andersen Windows, How to measure windows, read 24 October 2026.

3. Measure the width from inside, jamb to jamb, near the top, at the center and near the bottom, rounding down to the nearest 1/8 inch as Andersen says. Keep the smallest.

   Source: Andersen Windows, How to measure windows, read 24 October 2026.

4. Measure the height from head to sill at the left, center and right, to the nearest 1/8 inch, and keep the smallest.

   Source: Andersen Windows, How to measure windows, read 24 October 2026.

5. If an insert is the plan, measure the jamb depth between the stops. Andersen: "The window must also have a minimum jamb depth (space in between inside and outside stops) of 3 ¼ inches."

   Source: Andersen Windows, How to measure windows, read 24 October 2026.

6. Have your helper read each number back. Marvin: "To avoid a mistake, after you read the measurements, have them read the numbers back as you double check the dimensions."

   Source: Marvin, How to Measure for Replacement Windows and Doors, read 24 October 2026.

7. Label each window by room and position. Andersen suggests noting "The types of windows in your home (double-hung, casement, picture, etc.)." and "Any damage to or issues with your windows."

   Source: Andersen Windows, How to measure windows, read 24 October 2026.

Hire a pro when the measuring turns up one of these, or when the old window has to come out of a house built before 1978. Andersen's full-frame removal steps include "Remove the window stops.", "Remove the trim and casings." and "Remove the sill and frame, leaving the rough opening visible.", and EPA says a home built before 1978 may have lead-based paint.

Source: Andersen Windows, Full-Frame Window Replacement, read 24 October 2026.
Source: EPA, Lead Renovation, Repair and Painting Program: Do-It-Yourselfers, read 24 October 2026.

• The opening is out of square. Andersen: "If your windows are out of square, you’ll need to consider sizing adjustments when ordering new windows. Repairs to the rough opening might be needed as well. For this job, you’ll want an experienced window contractor."

  Source: Andersen Windows, How to measure windows, read 24 October 2026.

• The frame has rot in the trim, casing or sill, or you want a new size or shape: both are on Andersen's list of reasons for full-frame replacement, quoted in the section on inserts above.

  Source: Andersen Windows, Full-Frame Window Replacement, read 24 October 2026.

• The house was built before 1978. EPA's page for do-it-yourselfers says: "As a reminder, DIY projects can easily create dangerous lead dust, so EPA recommends homeowners hire lead-safe certified contractors for all RRP projects in pre-1978 homes. If you decide to hire a contractor to work on your pre-1978 home, the RRP Rule requires them to be lead-safe certified."

  Source: EPA, Lead Renovation, Repair and Painting Program: Do-It-Yourselfers, read 24 October 2026.

• You do not know when the house was built. EPA: "If you are unable to determine the age of your home, it’s safest to assume the area you are renovating has lead-based paint and use the DIY lead-safe work practices described below."

  Source: EPA, Lead Renovation, Repair and Painting Program: Do-It-Yourselfers, read 24 October 2026.

• The window is in a bedroom, a bathroom or low on a wall. Andersen says that if your home has older windows, your new windows might need to meet certain requirements that the old windows did not because of changes to the building code, for example "Sometimes tempered glass is required in bathrooms, with a low-placed window, or for transom windows above doors." and "Sometimes a bedroom might require a larger window in order to meet egress requirements (or windows that open wide enough to allow for an emergency escape)."

  Source: Andersen Windows, How to measure windows, read 24 October 2026.

The federal lead rule's definitions section, 40 CFR 745.83, defines minor repair in these words: "Minor repair and maintenance activities are activities, including minor heating, ventilation or air conditioning work, electrical work, and plumbing, that disrupt 6 square feet or less of painted surface per room for interior activities or 20 square feet or less of painted surface for exterior activities where none of the work practices prohibited or restricted by § 745.85(a)(3) are used and where the work does not involve window replacement or demolition of painted surface areas."

Source: eCFR, 40 CFR Part 745 Subpart E (Residential Property Renovation), read 24 October 2026.

The same rule lists, among renovations, "the removal of building components (e.g., walls, ceilings, plumbing, windows)".

Source: eCFR, 40 CFR Part 745 Subpart E (Residential Property Renovation), read 24 October 2026.

EPA's own pages put the household side plainly. On who the rule covers: "Generally speaking, the RRP Rule does not apply to homeowners doing RRP projects in their own home." On how common old paint is: "Approximately three-quarters of the homes in the United States built before 1978 still contain some lead-based paint." And on windows: "Old or worn lead-based paint, especially on surfaces subject to constant friction like doors or window sills, can produce chips and dust that become a hazard when inhaled or ingested."

Source: EPA, Lead Renovation, Repair and Painting Program: Do-It-Yourselfers, read 24 October 2026.
Source: EPA, Lead Renovation, Repair and Painting Program, read 24 October 2026.

What to ask for in writing if you hire:

• Whether the bid is for an insert or a full-frame replacement. Andersen says "Insert windows preserve the original frame, exterior trim, exterior siding, and interior casing." and, for full-frame, "The entire window frame and sash are removed".

  Source: Andersen Windows, Insert Window Replacement Installation, read 24 October 2026.
  Source: Andersen Windows, Full-Frame Window Replacement, read 24 October 2026.

• Whose measurements the order uses, and that the installer will measure again before ordering, the way Marvin describes its own process: "a friendly and knowledgeable window and door expert will confirm all measurements before ordering your new Marvin windows and doors."

  Source: Marvin, How to Measure for Replacement Windows and Doors, read 24 October 2026.

• For a pre-1978 house, the lead pamphlet: the federal rule says that "No more than 60 days before beginning renovation activities in any residential dwelling unit of target housing, the firm performing the renovation must:" "Provide the owner of the unit with the pamphlet".

  Source: eCFR, 40 CFR Part 745 Subpart E (Residential Property Renovation), read 24 October 2026.

• The NFRC ratings of the windows quoted, and the climate zone they were picked for; ENERGY STAR says its performance criteria "vary for each of the climate zones shown in the map above".

  Source: ENERGY STAR, What makes it ENERGY STAR? (residential windows, doors and skylights), read 24 October 2026.

• Which code items apply to each opening, such as tempered glass in a bathroom or egress in a bedroom.

  Source: Andersen Windows, How to measure windows, read 24 October 2026.

What should you write down before you order windows?

A complete record for each window is a room label, the two diagonals, three widths, three heights, the smallest of each, the jamb depth and the frame condition. Andersen also suggests noting the window types, how many there are and any damage, and says this information will help you communicate your needs when window shopping. Use this list for every opening:

Source: Andersen Windows, How to measure windows, read 24 October 2026.
Source: Andersen Windows, Full-Frame Window Replacement, read 24 October 2026.

1. Room and position, for example the left window in the back bedroom.
2. Window style, such as double-hung, casement or picture, the examples Andersen gives.

   Source: Andersen Windows, How to measure windows, read 24 October 2026.

3. Both diagonals, each to the nearest 1/8 inch, and whether they fall within 1/8 inch of each other.

   Source: Andersen Windows, How to measure windows, read 24 October 2026.

4. Width at the top, center and bottom, and the smallest of the three.

   Source: Andersen Windows, How to measure windows, read 24 October 2026.

5. Height at the left, center and right, and the smallest of the three.

   Source: Andersen Windows, How to measure windows, read 24 October 2026.

6. Jamb depth between the stops, against Andersen's minimum of 3 ¼ inches for an insert.

   Source: Andersen Windows, How to measure windows, read 24 October 2026.

7. Frame condition, since Andersen's full-frame page lists "There is rot in the window frame: trim, casing, or sill." among its reasons for full-frame replacement.

   Source: Andersen Windows, Full-Frame Window Replacement, read 24 October 2026.

8. The year the house was built, since EPA says a home built before 1978 may have lead-based paint.

   Source: EPA, Lead Renovation, Repair and Painting Program: Do-It-Yourselfers, read 24 October 2026.

9. Which rounding rule each figure follows: width rounded down to the 1/8 inch as Andersen says, or height rounded up to the inch as Marvin's quote guide says.

   Source: Andersen Windows, How to measure windows, read 24 October 2026.
   Source: Marvin, How to Measure for Replacement Windows and Doors, read 24 October 2026.

Who sees your contact details when you post on InstaBids?

Contact details stay hidden until the homeowner picks a contractor and they confirm the connection.

Source: InstaBids process and privacy, live-page excerpts, read 24 October 2026.

Sources for the facts on this page

• Andersen Windows, How to measure windows, read 24 October 2026
• Andersen Windows, Insert Window Replacement Installation, read 24 October 2026
• Andersen Windows, Full-Frame Window Replacement, read 24 October 2026
• Marvin, How to Measure for Replacement Windows and Doors, read 24 October 2026
• EPA, Lead Renovation, Repair and Painting Program: Do-It-Yourselfers, read 24 October 2026
• EPA, Lead Renovation, Repair and Painting Program, read 24 October 2026
• eCFR, 40 CFR Part 745 Subpart E (Residential Property Renovation), read 24 October 2026
• ENERGY STAR, What makes it ENERGY STAR? (residential windows, doors and skylights), read 24 October 2026
• InstaBids process and privacy, live-page excerpts reviewed 24 October 2026.