Calculate your total roof surface area from the building footprint and pitch — get square footage, roofing squares and a slope-adjusted total for material estimates.
Enter your footprint and pitch to get total roof area.
Your roof covers more surface than the ground beneath it because it’s sloped. To get the true area, take your building’s footprint (length × width) and multiply by the slope factor for your pitch. A 40 × 30 ft home (1,200 sq ft footprint) with a 6:12 pitch has a slope factor of 1.118 — so roughly 1,342 sq ft of actual roof before any waste.
Roofers then add a waste allowance (usually 10%, more for cut-heavy hip roofs) for trimming, overlaps and mistakes, and express the total in squares — one square = 100 sq ft of roofing.
Multiply your footprint by the slope factor for your pitch to get roof area.
| Pitch (rise:run) | Angle | Grade | Slope factor | Category |
|---|---|---|---|---|
| 1:12 | 4.8° | 8% | 1.003 | Low-slope |
| 2:12 | 9.5° | 17% | 1.014 | Low-slope |
| 3:12 | 14.0° | 25% | 1.031 | Conventional |
| 4:12 | 18.4° | 33% | 1.054 | Conventional |
| 5:12 | 22.6° | 42% | 1.083 | Conventional |
| 6:12 | 26.6° | 50% | 1.118 | Conventional |
| 7:12 | 30.3° | 58% | 1.158 | Conventional |
| 8:12 | 33.7° | 67% | 1.202 | Conventional |
| 9:12 | 36.9° | 75% | 1.250 | Conventional |
| 10:12 | 39.8° | 83% | 1.302 | Steep |
| 12:12 | 45.0° | 100% | 1.414 | Steep |
Once you know your roof area in squares, you can estimate materials and cost. Plug your numbers into a cost calculator by material — the Flat Roof Calculator for low-slope membranes, or Roof Replacement Cost Calculator, Metal, Asphalt Shingle and more for pitched roofs.
Here is exactly what the calculator does with your measurements.
Last reviewed: October 2026.
Multiply your building footprint (length × width) by the slope factor for your roof pitch, then add a waste allowance (typically 10%). This calculator does all three steps and also gives you the total in roofing squares.
A roofing square equals 100 square feet of roof surface. Roofers order and price materials by the square, so a 2,000 sq ft roof is 20 squares.
Always the sloped area, not the footprint — that’s the surface the roofer actually covers. This tool applies the slope factor for you so you don’t undercount.
Add about 10% for a simple gable roof and 15% or more for hip roofs and roofs with many valleys, dormers or penetrations, which require more cuts.
Once you have your numbers, compare free quotes from up to 3 licensed local roofers. No pressure — the choice is yours.