---
title: "Out of square window seat: when measurements don't add up"
url: "https://cushionguide.com/answers/cushion-measurements-dont-add-up-not-square/"
section: "The Cutting Table"
author: "Nate Kowalski, The Pattern Engineer of Detroit"
author_url: "https://cushionguide.com/masters/nate/"
published: "2026-01-26"
updated: "2026-01-26"
keywords: "measuring an out of square window seat, window seat measurements don't add up, out of square bay window cushion, measure diagonals window seat, custom window seat cushion tolerance"
---

# What if my window seat measurements don't add up because the corners aren't square?

*Old houses don't do right angles, partner. The drawing gets checked before the fabric does, and the check has a number.*

Also asked as: our house is 120 years old; not sure how to do the second diagonal

> **The quick answer:** If your window seat measurements don't add up, nothing is cut: the workshop draws the cushion to scale, checks that the shape closes, and asks you to re-measure named edges, measure both diagonals or fill in a lettered outline. A drawing that closes within 1/4 in (6 mm) is accepted, and production waits for your confirmation.

![Cream bouclé trapezoid cushion on a built-in bay window seat with two striped pads, olive branches and arched windows](https://cushionguide.com/media/bay-window-cushions/cream-trapezoid-bay-seat-striped-pads-arched-windows-928.webp "The seat pad follows the angled bay; two striped pads on top bring in mustard, sage and blue.")

## Key facts

- Every shaped cushion is drawn to scale and checked for closure before cutting.
- A drawing that closes within 1/4 in (6 mm) is accepted.
- Both diagonals, measured corner to corner, show whether the seat is square.
- The workshop can send a blank outline with each edge lettered A, B, C.
- With imperfect corners, the cut follows the long edge, depth and end sections; the slanted edges follow the geometry.
- Depth can be set separately for each section.
- Nothing goes into production until you confirm.

## Reply one: nothing is cut until the drawing closes

Before any shaped cushion is cut, the workshop draws it to scale and checks that the outline closes. If it doesn't, the reply names the edges in question. A drawing that closes within 1/4 in (6 mm) is accepted as is.

Angles are just lengths in disguise; a wrong angle usually means one wrong length.

## Reply two: what might you be asked to send?

1. **Re-measure the named edges**: The reply says which ones.
2. **Measure both diagonals**: Corner to corner, both ways. See [diagonal measurement](https://cushionguide.com/glossary/#diagonal-measurement) in the glossary.
3. **Send clear photos**: One wide-angle shot of the whole seat, plus a sharp top-down photo of your sketch, or the numbers typed as text.
4. **Fill in the lettered outline**: A blank drawing with each edge marked A, B, C; you write a number beside each letter.

## A 112 in seat whose slants wouldn't agree

A long angled seat came in with a 112 in long edge, 18 in of depth, and end sections of 32.5 in and 31 in. The slanted edges were written as 35 in. Drawn to scale, they came out at 37.2 in and 35.9 in.

The corners weren't square, so the cut followed the main dimensions, giving about 48.5 in on the short edge, and the slanted edges followed the geometry. The small differences went into the foam and the sewing tolerance.

## Follow-up: a 30 in and a 40 in figure that swapped places

On another angled seat the sides worked out to 34 in instead of the 40 in written down. The buyer went back with the tape and found a 30 in and a 40 in figure had traded places.

If your walls lean, you can also set the depth for each section. Run the numbers through the [bay window calculator](https://cushionguide.com/studio/bay-window-calculator/) first, and read [how to measure a window seat](https://cushionguide.com/guides/how-to-measure-window-seat-cushion/). Which corner of your seat do you trust least?

> **Tailor's margin:** Measure both diagonals of a window seat, corner to corner; if the two numbers differ, the corners aren't square and the cut follows the main edges.

> A shaped cushion drawing that closes within 1/4 in is accepted; a bigger gap means re-measuring before any fabric is cut.
>
> — Nate

## Buyer questions

**A common question:** My house is old and nothing is square. What if my measurements don't quite add up?

**The workshop's answer:** The drawing is checked for closure before cutting. If it doesn't close, you're asked to re-measure specific edges and both diagonals, or to fill in a blank outline lettered A, B, C. Within 1/4 in is accepted, and depth can be set per section.

**A common question:** Which number is the depth on an angled window seat?

**The workshop's answer:** Depth runs straight from front to back at 90 degrees at the deepest point, not along the angle. Length is the total between the outer corners.

**A common question:** How do I measure a bay window seat for a cushion?

**The workshop's answer:** Sketch the seat from above, write every edge (back, front, each angle) plus the depth, and type the numbers too. Upload the sketch and a photo.

## Questions buyers ask next

### Why compare the two diagonals of a seat?

In a true rectangle the two corner-to-corner diagonals are equal. If yours differ, the corners aren't square, and the cut follows the main edges while the slants follow the geometry.

### Can each section of a bay seat have its own depth?

Yes. With out-of-square walls you can set the depth you want for each section, and it is agreed before anything goes into production.

### Should I type my measurements or just send the sketch?

Both help. Replace a blurry sketch photo with a sharp one taken straight from above, or type the numbers with each edge named.

## Related at the atelier

- [Trapezoid Window Seat Cushion in Rust for a Small Stair Landing](https://cushionguide.com/readings/rust-trapezoid-landing-window-seat-cushion/): A small landing window seat feels generous when the cushion fills the whole trapezoid, wall to wall, with no bare board at the angled ends.
- [Window Seat Cushion Notched Around a Pier, Built for Long Sitting](https://cushionguide.com/readings/antique-gold-office-window-bench-cushion-pier-cutout/): A window seat cushion for long sitting should run the full bench, be notched around any pier or post, and stay near desk-chair height rather than sofa-soft.
- [Window Seat Cushion in Black Velvet: Thickness vs Seat Height](https://cushionguide.com/readings/black-velvet-l-shaped-window-seat-cushion-column-step/): For a window seat cushion, finished seat height equals the bench top plus the cushion thickness, so choose the thickness last.
- [Can you notch a cushion to fit around a window casing?](https://cushionguide.com/answers/cushion-with-notch-cut-out-around-casing/): Yes, a window seat cushion can be notched to clear a window casing so it sits fully against the wall.
- [How should I send measurements for an oddly shaped bench?](https://cushionguide.com/answers/measurements-for-odd-shaped-bench-cushion/): Send a photo of the space and a hand-drawn sketch on paper with every side measured and labeled A, B, C and so on.
- [Do you work in cm or inches, and which size will I get?](https://cushionguide.com/answers/cushion-size-cm-or-inches/): Either unit works, as long as you write it after every number, because the workshop confirms cm or inches before quoting.
- [How to measure a window seat cushion when the walls aren't square](https://cushionguide.com/guides/how-to-measure-window-seat-cushion/)
- [Measuring a 3-sided bay window seat cushion, panel by panel](https://cushionguide.com/guides/bay-window-seat-cushion-3-sided/)

*± 1/4 in — Nate*

---

Written by Nate Kowalski (https://cushionguide.com/masters/nate/) for CushionGuide (https://cushionguide.com/about/). The site explains made-to-measure cushions and the partner workshop makes them; measurements are in inches first. This Markdown mirrors the page at https://cushionguide.com/answers/cushion-measurements-dont-add-up-not-square/