Door Size Calculator
Work out a door's area and its rough opening — from the door size and the fitting gaps — in square metres, square feet and opening size.
A door is not its opening — the opening is the door plus the fitting gaps. This calculator gives you both: the door's area and the hole it needs.
Enter the door and the gaps; the opening follows.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Enter the door width and height.
- Enter the side and top gaps.
- Read the door area and the rough opening size.
Worked example
| Door | 0.9 m × 2.1 m |
|---|---|
| Area | 0.9 × 2.1 = 1.89 m² (20.34 ft²) |
| Rough width | 0.9 + 2 × 0.01 = 0.92 m |
| Rough height | 2.1 + 0.01 = 2.11 m |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
What is a rough opening?
The hole in the wall the door frame sits in — bigger than the door by the fitting gaps (10 mm each side and on top is common). The frame, not the door, fills the opening.
What are standard door sizes?
Common internal doors are 762 mm (30 in) or 838 mm (33 in) wide and 1,981–2,032 mm high; external doors are usually 915 mm (36 in) wide. Enter your own size here.
How much gap should I leave around a door?
About 10 mm each side and on top is a common fitting allowance, with a little more at the bottom for the floor covering. Adjust the gaps to your frame and floor.
Is this a substitute for building design?
No. This is a size calculation for planning. Structural requirements, accessibility and building-code compliance vary by location — see the safety notice.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.