The Pattern Room · a pattern with a calculator
How to measure a trapezoid cushion so the numbers close
A trapezoid is four numbers that have to agree with each other. Three of them decide the shape. The fourth, the slanted side, is the check most people skip, and the one that stops a cushion from being cut wrong.
SaveThe quick answer
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. Each slant must be longer than the depth: on a symmetric shape, slant = √(((front − back) ÷ 2)² + depth²). A 68 in front, 40 in back and 17 1/4 in depth give slants of about 22 1/4 in.
Key facts
- Front, back and straight depth fix a trapezoid; the slanted sides follow from them.
- A slanted side is always longer than the straight depth.
- Depth is measured straight at 90 degrees from the center of the back edge, never along a slant.
- Hand-measured slants often differ from the calculated value by about 1 in; larger gaps mean re-measuring every edge.
- Standard trapezoid thickness is 4 in (10 cm); trapezoids are ordered on the custom-shape listing with a drawing.
A 21 in side on a 17 1/4 in deep seat
A buyer in the US sent a trapezoid with a 68 in front, a 40 in back, a 17 1/4 in depth and a 21 in slanted side. Every number looked reasonable. Together they were impossible.
Trapezoid cushion measurements work like a triangle puzzle. Front, back and straight depth fix the shape; the slanted sides follow, and they are always longer than the depth. With those three numbers, each slant had to be about 22 1/4 in (56.5 cm). One of the four was wrong.
This Pattern is how a drawing gets checked before anything is cut, and how you can check yours first with a tape, a pencil and the trapezoid cushion calculator. Angles are just lengths in disguise.
Measuring a trapezoid seat in eight steps
Draw it from above and letter every edge
Sketch the seat as the ceiling sees it. Label the slanted sides A (left) and B (right), the front C, the back D and the straight depth E. Clipped corners or short straight ends get their own letters.
Pattern sheet Front 71 in, back 39.5 in, depth 21.5 in. The slanted sides work out at about 26.7 in. Measure the front edge (C)
Tape along the front face of the seat, corner to corner, in a straight line. Write it down in inches or centimeters, and write the unit.
Measure the back edge (D)
Along the wall or window side, corner to corner. In a bay, take it at the face of the window trim, not the glass. Record the true length even if a molding is in the way, and mark the molding on the drawing.
Measure the depth (E) at 90 degrees from the center
Start at the middle of the back edge and run the tape straight to the front edge, square to both. Not along a slant, not corner to corner. The corner of a framing square or a hardcover book keeps the tape honest. This is the most important number on the drawing.
Pattern sheet Front 71 in, back 39.5 in, depth 21.5 in. The slanted sides work out at about 26.7 in. Measure each slanted side on its own (A and B)
Straight from corner to corner, left and right, both written down. Don't copy one number to both sides; in one US bay they were 33 3/4 in and 33 in. Don't measure along an old cushion either: the tape follows its curve and reads about 1 to 1 1/4 in long.
In older rooms, add both diagonals
Measure front-left corner to back-right corner, then the other pair. Equal diagonals mean a symmetric shape; unequal ones show which way it leans. Old walls are rarely square, and diagonals let the maker draw what you actually have.
Run the right-triangle check
Half the difference between front and back is how far each side travels sideways: (68 − 40) ÷ 2 = 14 in. Then slant = √(14² + 17.25²) = √493.6 ≈ 22.2 in. A gap of about 1 in from your tape is normal hand measuring. A slant shorter than the depth, or a bigger gap, means re-measure every edge.
Pattern sheet Front 71 in, back 39.5 in, depth 21.5 in. The slanted sides work out at about 26.7 in. Add thickness and units, then order the custom shape
Write the thickness (4 in is standard) and the unit beside every number; one quote read inches as centimeters and came out far too low. Order on the custom-shape listing, choosing the range by the longest length and deepest depth, upload the drawing, and approve the maker's diagram before cutting.
Have it made
Measured it? Have Nate’s workshop make it
Every edge and both diagonals, to the nearest 1/4 in. If the numbers do not close, the workshop will say so on the call. — Nate
Leave your details and the workshop will call you to confirm measurements, fabric, foam and piping. No prices, dates or payment on this page: it is a request, not an order.
Worked trapezoids: do the numbers close?
| Front (C) | Back (D) | Straight depth (E) | Slants A and B | What happened |
|---|---|---|---|---|
| 68 in (173 cm) | 40 in (102 cm) | 17 1/4 in (44 cm) | about 22 1/4 in (56.5 cm) calculated | Written as 21 in; every edge re-measured |
| 72 1/2 in (184 cm) | 40 in (102 cm) | 17 1/2 in (44.5 cm) | about 24 in (61 cm) calculated | Measured 24 in on both sides; went to production |
| 115 in (292 cm) | 82 in (208 cm) | 20 in (51 cm) | about 26 in (66 cm) calculated | Sent without slants; the workshop calculated them and sent a diagram |
| 110 in (279 cm) | 68 in (173 cm) | 23 in (58 cm) | about 31 in (79 cm) calculated | Written as 32 in; a normal 1 in hand-measuring gap |
| 106 in (269 cm) | 56 in (142 cm) | about 24 1/2 in (62 cm) calculated | 35 in (89 cm) measured | Worked backward: the depth follows from the slants |
Let the calculator work out the slant lengths
The trapezoid cushion calculator takes front, back and depth and returns both slants in inches and centimeters. Put your measured slants beside its answer. If they disagree by more than about an inch, re-measure every edge before you order; a depth taken on a slight angle is one easy way to get there.
Two cases the simple formula doesn't cover. Straight ends first: if the shape has short straight returns at each end, subtract them from the depth before the triangle. A 90 in bottom, 52 in top, 27 in depth and 6 in straight ends travel 19 in across and 21 in up, so each slant is about 28 in (71 cm). Then lopsided shapes: measure each side's sideways offset on its own, which is what the diagonals are for.
For a bay with three or more panels, the bay window calculator handles each section, and the 3-sided bay Pattern walks through it panel by panel.
The bench that hid between two moldings
A buyer in the US measured a trapezoid bench that sat between wall moldings. The drawing said C 68 in, D 40 in, depth 17 1/4 in and one slant of 21 in. The workshop ran the triangle: those numbers wanted slants of about 22 1/4 in.
The reason came out in the next message. The buyer had shaved some sizes down to clear the moldings, trying to be helpful. The workshop asked for all five edges again, measured true.
The new set read A and B 24 in, C 72 1/2 in, D 40 in, depth 17 1/2 in. Half of 32 1/2 is 16 1/4, and √(16.25² + 17.5²) ≈ 23.9. That closes within a quarter inch, and it went to production. Give the maker the room as it is, partner; clearances are the maker's job.
Why a trapezoid can't ride on the rectangle listing
Rectangles cost less, and one angled end looks like a small thing. It isn't. If front and back lengths differ, or there is a notch or a curve, it's a custom shape. The rectangle listing never asks for a drawing, so nobody can check the geometry. Those orders get held, then cancelled and refunded for reordering, or sent a link for the price difference.
On the custom-shape listing, the size range you pick only sets the price bracket. The cushion is cut to your drawing, and 2, 3, 4 and 5 in thicknesses cost the same there. The custom-shape drawing Pattern covers labels and orientation.
Very long trapezoids, around 79 in (2 m) or more on the long side, may be suggested in two pieces; one 160 in bay was. Two sections are easier to make and handle. The trapezoid cushions and bay window cushions atlases show shapes others have ordered.
What does your front edge measure?
Common mistakes that stop a trapezoid at the cutting table
- A slanted side written shorter than the depth. Geometrically impossible, so every edge gets re-measured.
- Depth measured along the slant instead of straight at 90 degrees from the center.
- One slant copied to both sides. Left and right are rarely identical in real rooms.
- Short straight returns left off the sketch, so the pieces don't add up to the long edge.
- Letters on the sketch pointing to different edges than the maker's diagram. Check the approval drawing edge by edge.
- No units on the drawing. Inches read as centimeters change a quote dramatically.
- Sizes shaved down to clear a molding. Send true numbers and mark what is in the way.
- A trapezoid ordered as a rectangle, with no drawing uploaded.
Front, back and straight depth fix a trapezoid cushion: a 68 in front, 40 in back and 17 1/4 in depth force each slant to about 22 1/4 in.
Nate Kowalski
Buyer questions
- A common question
Which measurements do you need for a trapezoid bay window cushion?
The workshop's answerEvery edge: the front, the back (window) edge, both angled sides, any clipped corners or short end sides, and the depth. Measure the depth straight across at the deepest point, and measure each angled side separately, corner to corner, rather than repeating one number, because a slant is never the same as the depth. For a bay, measure every breaking point, mark cut-outs such as a window handle, give the thickness, and send a photo of the space with a drawing labeled A, B, C, D. Very long trapezoids may be suggested in two pieces.
- A common question
How should I letter the sides on my trapezoid drawing?
The workshop's answerWrite a length on every edge with letters: A to D, plus E and F for clipped corners or end sides. One real example read A 6 in, B 6 in, C 68 in, D 34 in, E 23 in, F 23 in, with a 22 in depth in the middle. Inches or centimeters both work, on the workshop's lettered template or your own sketch. Choose the nearest size range, then send the drawing; the workshop checks that the geometry closes, calculates any missing edge and sends a full list to confirm before cutting. Standard thickness is 4 in.
- A common question
My cushion isn't a rectangle. How do I order it?
The workshop's answerThrough the separate custom-shape (trapezoid) listing, not the rectangle one. Pick the size range by the longest length and the deepest depth; that only sets the price bracket. At checkout, upload a drawing with a measurement on every edge and a photo of the seat if you have one, and put the thickness and fabric code in Add Personalization. The cushion is cut to the drawing, not the range. If unsure, message the drawing first and the makers will say which options to select.
- A common question
Do I need to make a full-size paper template and mail it to you?
The workshop's answerNo. A hand drawing with every edge measured, or photos plus measurements, is enough for angled shapes. If you do make a full-size template, photograph it with the measurements marked and send it in the chat instead of mailing it; for curves it is the best starting point. The workshop checks that your numbers reconcile: a 106 in front, 56 in back and two 35 in sides give a depth of about 24 1/2 in.
- A common question
Can the corners of a cushion be cut off at an angle?
The workshop's answerYes. Clipped corners, such as 3 in off two corners or a back corner removed on a window ledge, go through the custom-shape listing, sized by the widest depth and the longest length. Show on the drawing how far each corner cuts in along both sides, and the corners are cut exactly to it. Main fabric and piping codes go in the personalization notes.
Trapezoid measuring, answered to the quarter inch
Do I need to know the angles of my trapezoid?
Not for a simple trapezoid. Front, back, straight depth and both slants describe the shape completely, and the angles follow from them. For multi-panel bays, note angles if you know them; many bay returns are around 135 degrees, but measure yours rather than assume.
Can the left and right slanted sides be different lengths?
Yes, and in real rooms they often are. One US bay seat measured 68 1/4 in at the back, 113 3/4 in at the front, 33 3/4 in on the left and 33 in on the right. Label each side separately and add both diagonals so the maker can draw the lean.
How close to my drawing will a trapezoid cushion come out?
When corners are out of square, the workshop cuts from the edges that sit against the walls and lets the diagonals follow, accepting a result within about 1/4 in. Every cushion is measured after sewing, and you can ask for photos showing the measurements before it ships.
What thickness suits a trapezoid window seat?
Four inches (10 cm) is the standard trapezoid thickness, and 2, 3, 4 and 5 in cost the same on the custom-shape listing. In a multi-cushion order, write the thickness separately for each piece. The window seat thickness Pattern compares the options.
Something sticks out into my seat. How do I measure around it?
Measure the true edges anyway, then mark the post, molding or window handle on the drawing with its size and position. The workshop draws a template for your approval. Shrinking the numbers yourself to clear an obstacle is how slants stop adding up.
