---
title: "5-Sided Bay Window Cushions: Angles, Sections and Seams"
url: "https://cushionguide.com/guides/5-sided-bay-window-cushion-calculator/"
section: "The Pattern Room"
author: "Nate Kowalski, The Pattern Engineer of Detroit"
author_url: "https://cushionguide.com/masters/nate/"
published: "2026-06-29"
updated: "2026-09-24"
keywords: "5 sided bay window cushion, multi angle bay window seat cushion, bay window cushion calculator, bay window seat cushion sections, hexagonal window seat cushion"
---

# 5-sided bay window cushions: working out angles, sections and seams

*A bay seat looks five-sided from the doorway and measures six or seven sides with a tape. Letter every edge, let the arithmetic check you, and decide where the seams go before anyone cuts foam.*

> **The quick answer:** Measure a 5-sided bay window cushion edge by edge: every back segment, the short end returns, the front and the depth at each break, each with its own letter. Each wing's run is √(side² − rise²), and the center back plus every run must equal the front within 1/4 in (6 mm). Long bays are often made as two or three sections.

![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.")

## Count the edges, not the windows

A **5 sided bay window cushion** almost never has five sides. Five windows, maybe, but the cushion also meets two short end walls and runs along a straight front, and every one of those edges gets cut.

One buyer in the US asked about a trapezoid, measured two more edges of 5 in (12.7 cm) and found a hexagon. Another sent seven letters, A to G. Every multi angle bay window seat cushion starts the same way, including a 5-window bay 115 7/8 in (294 cm) long and 16 in (40.5 cm) deep.

Below is the arithmetic the workshop runs before cutting. The shapes live under [bay window cushions](https://cushionguide.com/cushions/bay-window-cushions/); three-window bays have [their own Pattern](https://cushionguide.com/guides/bay-window-seat-cushion-3-sided/).

## Let the bay window calculator draw it first

The [bay window cushion calculator](https://cushionguide.com/studio/bay-window-calculator/) takes 3, 5 or 7 wall sections, measured left to right along the back of the seat board, plus the corner angles, the bay's projection or the opening width. Pick a straight front or a seat that follows the bay, tick 'split at the corners' for sections, and it draws the plan to scale with every piece listed.

Treat it as a second opinion, partner. If the plan doesn't look like your seat, one of your numbers is wrong, and a screen is a cheaper place to find that than foam.

## From tape measure to lettered drawing in seven moves

1. **Measure from the trim, not the glass**: Measure along the seat board's back edge, from the face of the window trim. The glass sits further back and adds inches the cushion can't use.
2. **Letter every edge, left to right**: End return, each window segment, the other end return, then the front. Five windows with returns make eight edges; the depth gets its own letter too.
3. **Take the depth at every break**: Measure straight back from the front edge at 90 degrees to each corner where the window turns. Where a section's two ends disagree, use the smaller figure.
4. **Measure left and right separately**: One US trapezoid measured 33.75 in (85.7 cm) on the left and 33 in (83.8 cm) on the right. Never copy one side to the other.
5. **Turn each wing into a run and a rise**: The rise is how much deeper the seat gets across a wing. The run is √(side² − rise²); the wing's angle off the front is arctan(rise ÷ run). Angles are just lengths in disguise.
6. **Check that the numbers close**: With square end returns, the center back plus every run must equal the front, within 1/4 in (6 mm). One buyer's sides came out 34 in, not 40, because a 30 and a 40 had traded places. Out of square? Add both [diagonals](https://cushionguide.com/glossary/#diagonal-measurement).
7. **Type the list, then approve the drawing**: Send the sketch, a photo and every letter typed out: 'A 4.3, B 4.3, C 78.5 in'. Sketches blur and attachments fail. The workshop draws it to scale and holds production until you approve.

## Three real bays, run through the arithmetic

Three seats from past orders, all with short end returns. For a symmetrical seat: run = (front − back) ÷ 2, rise = depth − end, side = √(run² + rise²). The window corners come out at 133°, 129° and 139°, near the common 135° but never on it, so don't assume 45° returns.

| Bay seat | Front | Back | Ends | Depth | Sides written | Sides by the math | Wing angle |
|---|---|---|---|---|---|---|---|
| Hexagon, first called a trapezoid | 78 in (198 cm) | 46.5 in (118 cm) | 5 in (12.7 cm) | about 22 in (56 cm) | 23 in | 23.2 in | 47° |
| Angled top, A to G | 78.5 in (199.5 cm) | 48 in (122 cm) | 4.3 in (11 cm) | 23 in (58.5 cm) | 24 in | 24.1 in | 51° |
| Wide bay | 104 in (264 cm) | 57 in (145 cm) | 4 in (10 cm) | 24.5 in (62 cm) | 31 in | 31.2 in | 41° |

Sides rounded to 0.1 in, angles to the nearest degree. All three close within 1/4 in (6 mm).

## The trapezoid that turned out to be a hexagon

A buyer in the US asked whether corduroy would work on a trapezoid bay seat. Then, measuring properly, they found two short extra edges where the seat met the side walls. The trapezoid was a hexagonal window seat cushion.

The workshop turned the sketch into a template: 78 in (198 cm) front, 46.5 in (118 cm) back, two angled sides of 23 in (58.5 cm), two end sides of 5 in (12.7 cm), about 22 in (56 cm) deep. Order by the size ranges that fit 78 in and 22 in, it said, and write every edge into personalization. The fabric: navy [corduroy](https://cushionguide.com/fabrics/corduroy/).

The home check: 15.75 in of run, 17 in of rise, 23.2 in of side. The tape said 23. The shape was honest, and the corduroy question was the easy part.

## One piece, or split at the angles?

Any number of angles can be one continuous cushion; a bay 98.5 in (250 cm) across the front has been. Whether yours should be decides your **bay window seat cushion sections**.

**Split** long seats and lidded ones. A trapezoid around 79 in (200 cm) long is usually made as two cushions, because angled sides can make one long piece bend or shift; a 160 in (406 cm) bay got the same advice. Lids get a cushion each; see [hinged storage lids](https://cushionguide.com/guides/storage-bench-cushion-hinged-lid/).

**Split at the angles** when the seat follows the bay all round: three cushions for a wrap-around bay is the easier build.

**Keep it whole** if a seam bothers you more than the handling, and ask before ordering.

Either way, the fabric roll is about 55 in (140 cm) wide, so oversized panels get seams toward the back, and a stripe may only run one way ([stripe direction](https://cushionguide.com/guides/striped-cushion-stripe-direction/)). The [zipper](https://cushionguide.com/glossary/#hidden-zipper) hides on the back edge or underneath.

## Why bay numbers refuse to close

- ✗ **An angled side shorter than its rise.** Impossible: a slanted edge is always longer than the depth it spans.
- ✗ **End returns left off.** A 4 or 5 in return turns a trapezoid into a hexagon.
- ✗ **No depth on the sketch.** One order paused for the front-to-back depth before its 91 in longest length meant anything.
- ✗ **Letters pointing at different edges.** Your A must be the maker's A; type 'A = left end return'.
- ✗ **A bay on the rectangle listing.** If checkout doesn't ask for a drawing, you're on the wrong listing.

> **Tailor's margin:** On a bay window seat with square end returns, the center back plus every wing's run must add up to the front edge, within 1/4 in.

> A bay window seat with two 5 in end returns is a hexagon, not a trapezoid. Measure every edge the cushion touches, then the front.
>
> — Nate Kowalski

## Buyer questions: bays with more than three sides

**A common question:** Our bay has five windows and a lot of angles. Can you make a cushion for it?

**The workshop's answer:** Yes. 5-window, pentagon and hexagon seats, and bays where each window sits at a slightly different angle, are made regularly. Send a sketch with every segment, any angles you know and the depth at each breaking point; it becomes a template for you to check before you order.

**A common question:** On an angled window seat, which measurement counts as the depth?

**The workshop's answer:** The straight distance from front edge to back edge at 90 degrees, at the deepest point, not along the angle. It's the most important number on the template. Length is the total between the outer corners.

**A common question:** My walls aren't square and my numbers don't add up. What happens now?

**The workshop's answer:** The seat is drawn to scale and checked for closure. If it doesn't close, you re-measure specific edges, measure both diagonals or fill in a lettered outline. A gap within 1/4 in is accepted, and nothing is cut until you confirm.

**A common question:** The bay seat has lift-up lids. Should the cushion be split to match?

**The workshop's answer:** Yes. Cushions over flip-up lids are divided at the lid lines, one per lid. Over a radiator, a 106 x 15 in seat became two 53 x 15 in cushions. Sections that differ, even by about 1.5 in in depth, each need their own drawing.

## Bay arithmetic, answered short

### What angle is a typical bay window corner?

Many read about 135° inside the seat, a 45° bay, but real ones wander: the three seats on this page ran 129° to 139°. Measure or calculate yours.

### Where does the zipper go on a bay window cushion?

Every foam cushion has a zippered, removable cover, hidden on the back edge against the window or underneath; you choose. On long covers it often wraps two corners so the foam slides out. See [zipper placement](https://cushionguide.com/guides/cushion-zipper-placement/).

### What fabric suits a sunny bay window seat?

A sun-resistant one. A buyer with a black 160 x 24 in (406 x 61 cm) trapezoid bay asked for exactly that, and the workshop stocks them. More in [window seat cushion fading](https://cushionguide.com/guides/window-seat-cushion-fading-sun/).

### Does a bay cushion have to be the same depth all the way round?

No. Depth can be set per section when walls are out of square, and a straight-front seat is deeper at the center than at the ends. Give the depth at every break.

## Related at the atelier

- [Big bays, long seats: where to split a large bay window cushion](https://cushionguide.com/concepts/large-bay-window-cushions/): A large bay window cushion longer than about 7.5 ft (2.3 m) is usually best made in two or more pieces, joined at a bay corner or at the midpoint.
- [Bay Window Cushion in Dusty Rose Linen, Cut to the Angle](https://cushionguide.com/readings/dusty-rose-bay-seat-cushion-dressing-room/): A bay window cushion starts with a drawing, not a fabric: mark every side length and both angled ends, and the quote comes back exact.
- [Bay Window Cushion Piping, Read on a Five-Sided Bouclé Seat](https://cushionguide.com/readings/rust-boucle-five-sided-bay-window-cushion/): On a five-sided bay window cushion, self piping is the safe choice: it defines every facet by shape while keeping one color around all the angles.
- [Bay Window Cushion in Flax Linen: Wine Piping in Changing Light](https://cushionguide.com/readings/flax-bay-window-cushion-wine-piping-striped-lumbar/): Check a bay window cushion fabric in the bay itself at three times of day, because a three-panel bay lights the seat from several directions and the color shifts with them.
- [Cushions for Chicago bungalow bays, two-flats and porches](https://cushionguide.com/places/chicago-cushions/): A Chicago bungalow window seat cushion is a clipped trapezoid: a long straight front, a center window panel, two slanted sides and usually short straight returns at each end.
- [Can you make a cushion for a bay window with 5 windows, all at different angles?](https://cushionguide.com/answers/multi-angle-bay-window-cushion-5-windows/): Yes: a bay window seat cushion can have any number of angles, including 5-window, pentagon and hexagon bays.
- [Measuring a 3-sided bay window seat cushion, panel by panel](https://cushionguide.com/guides/bay-window-seat-cushion-3-sided/)
- [Window seat dimensions and the cushion sizes that fit them](https://cushionguide.com/guides/window-seat-dimensions/)

*± 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/guides/5-sided-bay-window-cushion-calculator/