Quick answer
Roof area = (building length + 2 × overhang) × (building width + 2 × overhang) × √(1 + (pitch ÷ 12)²). A 40 × 30 ft building with 1 ft overhangs and a 6:12 pitch has 1,502.6 ft² of roof, or 15.03 roofing squares.
Enter the building size, overhang and pitch. The calculator returns roof area, squares and the pitch factor.
Related tools: 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. Roof Sheathing Calculator: Sheets of plywood or OSB for a roof area.
How to use the Roof Area Calculator
- Enter building length (ft) (above 0).
- Enter building width (ft) (above 0).
- Enter overhang on every side (ft) (0 or more). It starts at 1.
- Enter pitch (rise per 12 in) (between 0 and 36).
- 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 |
| Overhang on every side | A number, in ft | ≥ 0, starts at 1 |
| Pitch (rise per 12 in) | A number | > 0, ≤ 36 |
Formula and method
Formula
area = (L + 2·overhang) × (W + 2·overhang) × √(1 + (pitch ÷ 12)²)
IEEE-754 double precision; shown to the decimals indicated.
Assumes a single uniform pitch over the whole footprint (gable or hip); dormers, valleys and cut-ups add area. Plan geometry only; confirm quantities with your supplier and local code before ordering.
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; Overhang on every side = 1; Pitch (rise per 12 in) = 6.
| Result | Value |
|---|---|
| Roof area | 1,502.6 ft² |
| Roofing squares | 15.03 |
| Rafter length factor | 1.118 |
Example 2
Inputs: Building length = 50; Building width = 24; Overhang on every side = 0; Pitch (rise per 12 in) = 8.
| Result | Value |
|---|---|
| Roof area | 1,442.2 ft² |
| Roofing squares | 14.42 |
| Rafter length factor | 1.2019 |
Example 3
Inputs: Building length = 36; Building width = 28; Overhang on every side = 1.5; Pitch (rise per 12 in) = 4.
| Result | Value |
|---|---|
| Roof area | 1,274.4 ft² |
| Roofing squares | 12.74 |
| Rafter length factor | 1.0541 |
The first line is the headline result (roof area). Below it the roof Area Calculator lists roofing squares and rafter length factor, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
The calculation
The footprint including overhang is 42 × 32 = 1,344 ft². At 6:12 the slope factor is 1.118, so the roof area is 1,344 × 1.118 = 1,502.6 ft², or about 15 squares (100 ft² each).
Simple roofs only
The formula suits simple gable or hip roofs. Dormers, valleys and cut-up roofs need more material. Add waste when ordering shingles: 10 to 15% is common.
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 | Roofing squares | Rafter length factor |
|---|---|---|---|
| 20 | 787.1 ft² | 7.87 | 1.118 |
| 40 | 1,502.6 ft² | 15.03 | 1.118 |
| 80 | 2,933.7 ft² | 29.34 | 1.118 |
| 160 | 5,795.9 ft² | 57.96 | 1.118 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Building length: entering "0" shows "Building length must be greater than 0." beside that field and no result.
- Pitch (rise per 12 in): entering "0" shows "Pitch (rise per 12 in) must be greater than 0." beside that field and no result.
Choosing between the Roof Area 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 |
|---|---|---|
| [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 |
| [Roof Sheathing Calculator](/construction/roof-sheathing-calculator/) | roof area, sheet size and waste allowance | Sheets needed |
| [Metal Roofing Calculator](/construction/metal-roofing-material-calculator/) | roof width to cover, panel coverage width and panel length | Panels needed |
| [Ice and Water Shield Calculator](/construction/ice-water-shield-calculator/) | area to cover, coverage per roll and waste allowance | Rolls 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
- Add waste when ordering shingles
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 building length and pitch (rise per 12 in) to confirm this.
Frequently asked questions
What is a roofing square?
100 square feet of roof.
Does a steeper roof have more area?
Yes, area rises with pitch.
Do I measure from the ground?
Use the building footprint plus overhangs.
What does the Roof Area Calculator calculate?
Roof surface area from the building footprint, overhang and pitch. It returns Roof area, Roofing squares and Rafter length factor.
What formula does the Roof Area Calculator use?
area = (L + 2·overhang) × (W + 2·overhang) × √(1 + (pitch ÷ 12)²). Assumes a single uniform pitch over the whole footprint (gable or hip); dormers, valleys and cut-ups add area.
What do I need to enter in the roof Area Calculator?
You enter Building length (ft), Building width (ft), Overhang on every side (ft) and Pitch (rise per 12 in).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Add waste when ordering shingles.
What is an example of a roof Area result?
With building length 40, building width 30, overhang on every side 1 and pitch (rise per 12 in) 6, the roof area is 1,502.6 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: Add waste when ordering shingles.
Related calculators
- 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.
- Roof Sheathing Calculator — Sheets of plywood or OSB for a roof area.
- Metal Roofing Calculator — Number of metal roofing panels across a roof width.
- Ice and Water Shield Calculator — Rolls of ice and water membrane for an area.
- Gutter Length Calculator — Total gutter length and number of sections from your eave lengths.
- Area Converter — Convert square metres, feet, acres, hectares and more.
Sources
- OpenStax, Prealgebra 2e (Rice University): perimeter, area, volume, circumference and the Pythagorean theorem
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI), Appendix B conversion factors (international foot 0.3048 m, inch 25.4 mm, US gallon 3.785411784 L)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.