---
title: "Trapezoid cushion slanted sides: measure both, trust the math"
url: "https://cushionguide.com/answers/measure-slanted-sides-trapezoid-cushion/"
section: "The Cutting Table"
author: "Nate Kowalski, The Pattern Engineer of Detroit"
author_url: "https://cushionguide.com/masters/nate/"
published: "2026-02-09"
updated: "2026-09-24"
keywords: "trapezoid cushion slanted side measurement, how to measure a trapezoid cushion, bay window cushion angled sides, trapezoid window seat cushion, trapezoid slant calculation"
---

# Do I need to measure the slanted sides of a trapezoid or bay window cushion?

*Yes, but not for the reason you think: the slanted sides are the check digit on your drawing. Here is how the workshop runs the numbers.*

Also asked as: How do I measure the angled sides?; Do both angled sides match?

> **The quick answer:** Yes, measure both slanted sides of a trapezoid cushion, because they can differ, but the workshop treats them as a check, not the master dimensions. The bottom length, top length, depth and any straight end pieces fix the slant. If your measured slant doesn't match the calculated one, another number is wrong, and you're asked to re-measure.

![Pebble gray tweed-look trapezoid window seat cushion with rust piping on white shaker drawers in a shiplap bay](https://cushionguide.com/media/trapezoid-cushions/gray-tweed-trapezoid-window-seat-cushion-rust-piping-1600.webp "The angled right end follows the bay wall, so the cushion fills every inch of the seat. A thin rust welt ties the gray tweed to the warm floorboards.")

## Key facts

- Measure both slanted sides separately; they can differ.
- Bottom, top, depth and any straight ends are the master dimensions.
- Slant = the diagonal of half the length difference and the depth minus the straight ends.
- A 90 in bottom, 52 in top, 27 in depth and 6 in ends give a slant of about 28 in (71 cm).
- A tape run along an old cushion reads 1 to 1.3 in (2.5 to 3.3 cm) long.
- Mismatched slants mean re-measuring every edge.
- If posts or moldings got in the way, a template is drawn for your approval.

## “Do I really need the slanted sides?”

Measure both, because the two sides of a real bay can differ. Then treat them as a check: bottom length, top length, depth and any straight end pieces fix the slant. Angles are just lengths in disguise, partner, and so are slants.

The [trapezoid cushions](https://cushionguide.com/cushions/trapezoid-cushions/) hub has more shapes, and the [trapezoid cushion calculator](https://cushionguide.com/studio/trapezoid-cushion-calculator/) runs the arithmetic.

## The check, run on a real bay seat

One bay seat from a past order: 90 in (229 cm) bottom, 52 in (132 cm) top, 27 in (69 cm) depth, 6 in (15 cm) straight ends.

1. **Halve the length difference**: (90 − 52) ÷ 2 = 19 in of lean per side.
2. **Take the straight ends off the depth**: 27 − 6 = 21 in of rise.
3. **Find the diagonal**: The square root of (19² + 21²) is about 28.3 in: a slant of about 28 in (71 cm).
4. **Compare it with the tape**: A written 24 in slant can't close this shape, so every edge gets measured again.

## “My two angled sides don't match. Now what?”

A 68 in (173 cm) front, 40 in (102 cm) back and 17.25 in (44 cm) depth give slants of about 22 1/4 in (57 cm), so a side written as 21 in flagged an error and a full re-measure.

Two habits cause these slips. A tape run along an old cushion follows its curve and reads long. And posts or moldings push the tape around them; say so, and a template is drawn for your approval. The [trapezoid measuring pattern](https://cushionguide.com/guides/trapezoid-cushion-measurements/) shows where to hold the tape.

> **Tailor's margin:** Measure a trapezoid cushion's slanted side along the seat itself, not an old cushion: a tape that follows the old curve reads 1 to 1.3 in long.

> On a trapezoid cushion, bottom, top and depth fix the slanted side, so a measured slant that disagrees means another number on the drawing is wrong.
>
> — Nate

## Buyer questions

**A common question:** Do I have to measure the slanted sides of my bay window seat?

**The workshop's answer:** Not strictly. The bottom, top, depth and any straight end pieces fix the slant, and it is calculated to check your numbers.

**A common question:** Are both slanted sides of a trapezoid cushion the same?

**The workshop's answer:** Not always, so measure both. Before cutting, front, back, depth and sides are checked to see that they close; if they don't, you're asked to re-measure every edge.

**A common question:** My bay seat has a hard maximum length. Can the cushion stay under it?

**The workshop's answer:** Yes. Handmade cushions can vary by about 1 in (2 to 3 cm) from the ordered size, so name the hard maximum and the cushion is kept under it.

## Questions buyers ask next

### Is the slanted side always longer than the seat depth?

Longer than the rise it spans, yes. Take any straight end pieces off the depth first; the slant is the long side of the right triangle that remains.

### What are straight ends on a bay seat drawing?

Short square sections at each end of the seat, running front to back before the angle starts, like the 6 in (15 cm) ends above. Subtract them from the depth first.

### Does the workshop cut my slant or the calculated one?

The shape follows the master numbers: bottom, top, depth and straight ends. The slant is calculated from them, and any mismatch is settled with you first.

## Related at the atelier

- [How to measure a trapezoid cushion so the numbers close](https://cushionguide.com/guides/trapezoid-cushion-measurements/): Trapezoid cushion measurements are the front edge, the back edge and the straight depth taken at 90 degrees, plus both slanted sides as a check.
- [Which measurement is the depth on an angled window seat?](https://cushionguide.com/answers/trapezoid-window-seat-depth-measurement/): Depth is the straight line from the front edge to the back edge at 90 degrees, taken at the deepest point, never along the angle.
- [Trapezoid Bench Cushion in Mudcloth on a Floating Oak Bay Seat](https://cushionguide.com/readings/mudcloth-trapezoid-cushion-floating-oak-bay-seat/): A trapezoid bench cushion in a bay reads well from across the room when its shape and contrast are right: angled ends that follow the side walls, a straight front edge, and a dark field against pale…
- [Trapezoid Bench Cushion in Green Stripe with a Solid Green Welt](https://cushionguide.com/readings/green-stripe-trapezoid-seat-cushion-green-welt/): An indoor striped trapezoid bench cushion borrows two habits from outdoor cushions: a solid welt in the stripe's darker color to hold the edge, and a box edge that keeps the angled end crisp.
- [Trapezoid Bench Cushion in Gray Tweed with Rust Piping](https://cushionguide.com/readings/gray-tweed-trapezoid-window-seat-cushion-rust-piping/): A trapezoid bench cushion in cool gray stays warm-looking when its piping takes its color from the floor or brickwork, not from the fabric.
- [How much does a custom trapezoid cushion cost?](https://cushionguide.com/answers/trapezoid-cushion-cost-quote/): A custom trapezoid cushion has no fixed price: it is quoted from the length of every side (back, front and both angled sides), the thickness, the fabric and whether it is part of a set.
- [Measuring a hexagon cushion, or an octagon, without a protractor](https://cushionguide.com/guides/hexagon-and-octagon-cushion-measuring/)

*± 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/measure-slanted-sides-trapezoid-cushion/