Quick answer
Roof area = footprint × √(1 + (rise ÷ 12)²), and one roofing square is 100 ft². A 40 × 30 ft building at 6:12 pitch with no overhang has 1,341.64 ft², or 13.42 squares.
Enter the building size, pitch and waste. The calculator returns the roof area and squares.
Related tools: Metal Roofing Calculator: Number of metal roofing panels across a roof width. Roof Pitch Calculator: Roof pitch as rise per 12 inches, angle, slope percent and rafter length factor. Roof Shingles Calculator: Squares and bundles of shingles for a roof area.
How to use the Roofing Calculator
- Enter building length (ft) (above 0).
- Enter building width (ft) (above 0).
- Enter roof pitch (rise per 12 in run) (between 0 and 24). It starts at 6.
- Optionally, enter waste allowance (%) (between 0 and 50). It starts at 10.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Building length | A number, in ft | > 0 |
| Building width | A number, in ft | > 0 |
| Roof pitch (rise per 12 in run) | A number | > 0, ≤ 24, starts at 6 |
| Waste allowance (optional) | A number, in % | ≥ 0, ≤ 50, starts at 10 |
Formula and method
Formula
roof area = footprint × √(1 + (rise ÷ 12)²); 1 roofing square = 100 ft²
IEEE-754 double precision; shown to at most 6 decimal places.
Pitch factor for a simple gable roof; overhangs, hips and valleys add area.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Building length = 40; Building width = 30; Roof pitch (rise per 12 in run) = 6; Waste allowance = 0.
| Result | Value |
|---|---|
| Roof area | 1,341.6408 ft² |
| Squares | 13.4164 |
Example 2
Inputs: Building length = 40; Building width = 30; Roof pitch (rise per 12 in run) = 6.
| Result | Value |
|---|---|
| Roof area | 1,475.8049 ft² |
| Squares | 14.758 |
The first line is the headline result (roof area). Below it the roofing Calculator lists squares, which describe the same case from other angles. IEEE-754 double precision; shown to at most 6 decimal places.
From footprint to slope
The footprint is 1,200 ft². A 6:12 pitch multiplies it by 1.118, giving 1,341.64 ft². This works because the sloped surface is longer than the horizontal span.
Simple shapes only
Dormers, hips and valleys add area. Use overhang by adding twice its width to each dimension, or use the Roof Area calculator, which includes the overhang.
How the result changes with building length
Holding every other input at the first worked example, this table changes only building length. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Building length | Roof area | Squares |
|---|---|---|
| 20 | 670.8204 ft² | 6.7082 |
| 40 | 1,341.6408 ft² | 13.4164 |
| 80 | 2,683.2816 ft² | 26.8328 |
| 160 | 5,366.5631 ft² | 53.6656 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Roof pitch (rise per 12 in run): entering "0" shows "Roof pitch (rise per 12 in run) must be greater than 0." beside that field and no result.
Choosing between the Roofing Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Metal Roofing Calculator](/construction/metal-roofing-material-calculator/) | roof width to cover, panel coverage width and panel length | Panels needed |
| [Roof Pitch Calculator](/construction/roof-pitch-calculator/) | i know the and value | Pitch |
| [Roof Shingles Calculator](/construction/roof-shingles-calculator/) | roof area, bundles per square and waste allowance | Bundles needed |
| [Rafter Length Calculator](/construction/rafter-length/) | horizontal run and pitch | Rafter length |
| [Roof Sheathing Calculator](/construction/roof-sheathing-calculator/) | roof area, sheet size and waste allowance | Sheets needed |
Background: construction estimates
Material quantities are geometry plus a margin. Calculate the exact volume or area, add an allowance for waste, spillage, cuts and uneven ground (calculator pages commonly mention 5 to 10 percent, a rule of thumb that varies by material and job) and round up to whole units. One cubic yard is exactly 27 cubic feet.
Suppliers often have minimum order sizes and may charge for small loads, so confirm quantities, product yields and local code with your supplier or a qualified estimator before ordering.
Assumptions and limitations
- Simple roof shapes only
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for roof pitch (rise per 12 in run) to confirm this.
Frequently asked questions
How many squares is my roof?
Divide the roof area by 100.
Does waste matter?
Yes, add 10 to 15% when ordering.
What pitch factor applies?
It depends on the pitch.
What does the Roofing Calculator calculate?
Roof surface area and roofing squares from the footprint and pitch. It returns Roof area and Squares.
What formula does the Roofing Calculator use?
roof area = footprint × √(1 + (rise ÷ 12)²); 1 roofing square = 100 ft². Pitch factor for a simple gable roof; overhangs, hips and valleys add area.
What do I need to enter in the roofing Calculator?
You enter Building length (ft), Building width (ft) and Roof pitch (rise per 12 in run). Optional: Waste allowance (%).
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Simple roof shapes only.
What is an example of a roofing result?
With building length 40, building width 30, roof pitch (rise per 12 in run) 6 and waste allowance 0, the roof area is 1,341.6408 ft².
Can I rely on this calculator for a decision?
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work. For this calculator in particular: Simple roof shapes only.
Related calculators
- Metal Roofing Calculator — Number of metal roofing panels across a roof width.
- Roof Pitch Calculator — Roof pitch as rise per 12 inches, angle, slope percent and rafter length factor.
- Roof Shingles Calculator — Squares and bundles of shingles for a roof area.
- Rafter Length Calculator — Common rafter length from run and pitch.
- Roof Sheathing Calculator — Sheets of plywood or OSB for a roof area.
- Ice and Water Shield Calculator — Rolls of ice and water membrane for an area.
Sources
- OpenStax, University Physics Volume 1 (Rice University): mechanics
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.