A bed sheet calculator works out the fabric dimensions you need for a mattress, typically returning the cut size for a flat sheet or the yardage for sewing your own. You enter the mattress length, width, and depth (plus your preferred tuck allowance), and it gives you the total fabric size in inches or centimeters, and often the yards or meters of cloth to buy. This guide explains how the math works, how to use it, and what to watch out for when you measure.
Formula
The flat-sheet formula is straightforward: you need the mattress top surface, plus two depths (one for each side), plus a tuck allowance on each side to go under the mattress. In symbols:
Cut length = L + 2D + 2T
Cut width = W + 2D + 2T
- L = mattress length (in inches or cm)
- W = mattress width
- D = mattress depth (thickness)
- T = tuck allowance per side (how much fabric you want to tuck under)
For a fitted sheet, the math is a bit different: you cut a rectangle, then remove squares from each corner equal to the depth plus hem allowance, and sew the sides together. The cut size for a fitted sheet is roughly (L + 2D + hem) by (W + 2D + hem), then you take out a square of side (D + hem) from each corner. Most calculators simplify by giving the flat-sheet size, since that is what you need for fabric purchasing.
Worked Example
Using the default values from the calculator above: a queen mattress 80 inches long, 60 inches wide, 10 inches deep, with a 6-inch tuck allowance per side.
Cut length = 80 + 2×10 + 2×6 = 80 + 20 + 12 = 112 inches
Cut width = 60 + 2×10 + 2×6 = 60 + 20 + 12 = 92 inches
For fabric yardage on a 54-inch-wide bolt: you need the length in yards, rounded up to the next full yard. 112 inches ÷ 36 = 3.11, so you buy 4 yards. The calculator shows 4 yards.
How to Measure Your Mattress
Measurements are the most common source of error. Use a metal tape measure, not a cloth one that can stretch. Measure the length and width at the center of the mattress, not the edge, because some mattresses taper slightly. For depth, measure from the top seam to the bottom seam, including any pillow top. If you have a mattress topper, include its thickness in the depth. Write down the numbers in inches or centimeters and stick to one unit throughout.
How to Choose a Bed Sheet Size
Most people do not sew their own sheets; they buy ready-made. The calculator still helps you understand what size label to look for. Standard mattress sizes are:
- Twin: 39 in × 75 in (99 × 190 cm)
- Full/Double: 54 in × 75 in (137 × 190 cm)
- Queen: 60 in × 80 in (152 × 203 cm)
- King: 76 in × 80 in (193 × 203 cm)
- California King: 72 in × 84 in (183 × 213 cm)
These are nominal; actual dimensions vary by brand. Always measure your own mattress. For deep mattresses (over 12 inches), look for sheets labeled “deep pocket� – they typically fit up to 15–18 inches. The calculator’s depth input tells you whether a standard sheet (usually 7–9 inches) will fit your mattress. If your mattress depth is more than the sheet’s pocket depth, the sheet will pop off.
Common Mistakes
- Mixing units: Inches and centimeters in the same formula. Always convert to one unit.
- Forgetting the depth: A 10-inch mattress needs 20 extra inches of length and width just for the sides.
- Not adding hem allowance: When sewing, you need extra fabric for hems – usually 1–2 inches per side. The calculator’s tuck allowance can include this, or you add it separately.
- Rounding too early: If you round each side to the nearest inch, you might end up short. Keep fractions until the end.
- Assuming the fabric width is enough: A 44-inch-wide fabric will not work for a queen width (92 inches) unless you piece it. Check the fabric width before buying.
Fabric Yardage and Widths
Fabric comes in standard widths: 44–45 inches, 54–60 inches, and sometimes 108–110 inches for sheeting. For a queen flat sheet, you need a width of about 92 inches, so a 90-inch-wide fabric will just barely work, but a 108-inch-wide fabric is more comfortable. If using 54-inch fabric, you would need to sew two panels together, which adds a seam down the middle – not ideal for a sheet. The yardage calculation assumes you can use the full width. For a king, you almost always need wide sheeting fabric.
Fitted Sheet vs. Flat Sheet
A fitted sheet has elastic corners and is shaped to the mattress. The cut size is similar to the flat sheet, but you remove a square from each corner. The square side equals the depth plus hem allowance (usually 1 inch). For the queen example: depth 10 + 1 = 11-inch squares cut from each corner. Then you sew the sides together and add elastic. The calculator gives the flat sheet size, which is also the starting point for a fitted sheet. If you are making a fitted sheet, add the hem allowance to the depth before cutting the corner squares.
Frequently Asked Questions
What if my mattress is an odd size, like a camper RV?
Measure the mattress exactly, then use the calculator with your numbers. RV mattresses often have rounded corners, so you may need to add a couple of inches to the depth to account for the curve. For a truly custom fit, make a paper template first.
How much tuck allowance should I use?
For a flat sheet, 6 inches per side is typical – enough to tuck under the mattress without pulling out. If you like a generous tuck, use 8–10 inches. For a fitted sheet, you do not need a tuck allowance; the elastic does the work.
Can I use the calculator for a duvet cover?
No, a duvet cover needs the duvet insert dimensions, not the mattress. The formula is similar but uses the duvet length and width plus seam allowance and a button/zipper overlap. Use a duvet-specific calculator.
Why does fabric yardage round up?
Fabric is sold by the full yard (or half-yard in some stores). You cannot buy 3.11 yards; you buy 4 yards. Rounding up ensures you have enough for cutting errors and hems.
What if my mattress depth is more than 24 inches?
The calculator allows up to 24 inches, which covers most mattresses. If you have a custom extra-thick mattress, you can still use the formula manually: just add the depth to the length and width. Deep-pocket sheets are available up to 22–24 inches, but beyond that you may need custom bedding.