Door Frame Calculator
Find rough opening size and casing length for a door.
Results update as you type.
How it works
Framing an opening for a new door starts with getting the rough opening right, and this door frame calculator gives you the dimensions and the trim length from just the door's width and height. A rough opening is the framed hole in the wall, and it must be larger than the door and its jamb so the frame can be set plumb, level, and square with shims. The standard allowance adds about two inches to the door's width and two and a half inches to its height, which accommodates a typical jamb thickness plus shim space; the calculator applies this rule and returns both rough dimensions. It also estimates the casing, the decorative trim that frames the door on the wall, by measuring around the top and two sides of the opening and converting to linear feet, which helps you buy the right amount of moulding. Carpenters, remodellers, and DIY builders use these numbers to frame walls and order materials without guesswork. Always double-check against the specific door and jamb manufacturer's instructions, since allowances can vary slightly by product and by whether the door is pre-hung or a slab. Enter the door width and height to see the rough opening and casing length.
rough width = door + 2 in; rough height = door + 2.5 in
The rough opening for a pre-hung door must be larger than the door itself to fit the jamb and leave room for shims that plumb and square the frame. The common rule adds two inches to the door width and about two and a half inches to the height. The casing, or trim, runs around three sides of the opening, so its length is twice the rough height plus the rough width, converted to feet.
Source:Standard door rough-opening allowances
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter door width and door height.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Rough opening width updates instantly as you type — no button to press.
Frequently asked questions
What is a rough opening?
A rough opening is the framed hole left in a wall's structure to receive a door or window. It is deliberately made larger than the unit itself so that the frame or jamb can be inserted and then adjusted with shims until it is perfectly plumb, level, and square. Getting the rough opening size right is the first step to a door that hangs and closes properly.
How much bigger than the door should the opening be?
A common rule of thumb is to make the rough opening about two inches wider than the door and roughly two to two and a half inches taller. This leaves room for the jamb thickness on each side and top, plus a shim gap to square the frame. Always confirm the exact figure with the door manufacturer, as pre-hung units may specify their own allowances.
How is the casing length calculated?
Casing trims the door on three sides, the top and the two verticals, so the calculator adds twice the rough opening height plus the width, then converts inches to feet. It is wise to buy a little extra to allow for mitre cuts at the corners and any waste. The result is the minimum linear footage of moulding you will need for one side of the opening.
Does this work for pre-hung and slab doors?
The rough opening allowance applies mainly to pre-hung doors, which come with the jamb already attached and need space for that jamb plus shims. A slab door, sold without a frame, is hung inside an existing or separately built jamb, so you would size the opening around that jamb. Use the calculator for standard pre-hung framing and adjust for special situations.
Should I account for flooring height?
Yes, in practice. The rough opening height should allow for the finished floor plus any threshold and an undercut gap beneath the door for clearance over carpet or tile. Many builders leave the bottom of the opening slightly higher to accommodate finish flooring installed later. Consider your floor build-up when framing so the door swings freely once everything is finished.