You can't price a roof until you know how big it is — and "how big" isn't your house's square footage. A 2,000 sq ft house does not have a 2,000 sq ft roof. The roof is bigger, because it's sloped, and it's measured in squares.
Here's the whole calculation in three steps.
Step 1: Find your roof's footprint (sq ft)
The footprint is the flat ground area the roof covers — roughly your home's length × width, plus the garage and any covered areas. For a simple rectangular home: length × width. (Don't use interior square footage — that misses overhangs and counts multiple floors.)
Step 2: Multiply by your roof pitch
Pitch (steepness) makes the actual roof surface larger than the footprint. Multiply the footprint by the factor for your pitch:
| Roof pitch | Multiply footprint by | Notes |
|---|---|---|
| Flat / low slope | × 1 | Almost no slope |
| Low (3/12–4/12) | × 1.12 | Gentle |
| Medium (5/12–8/12) | × 1.25 | Most common |
| Steep (9/12+) | × 1.41 | Hard to walk |
Not sure of your pitch? Most homes are "medium." Using × 1.25 is a safe default.
Step 3: Divide by 100 to get squares
Roof area ÷ 100 = squares. That's the number every roofer quotes from.
Worked example
A 1,600 sq ft footprint with a medium pitch: 1,600 × 1.25 = 2000 sq ft of roof ÷ 100 = 20.0 squares. At a typical architectural-shingle price, that's roughly $13,000–$15,000 installed.
Square feet to squares — quick reference
| Roof area (sq ft) | Squares |
|---|---|
| 1,500 sq ft | 15 squares |
| 2,000 sq ft | 20 squares |
| 2,500 sq ft | 25 squares |
| 3,000 sq ft | 30 squares |
| 4,000 sq ft | 40 squares |
Why the manual method is only an estimate
Hand-measuring misses valleys, dormers, hips, and overhangs, and most people guess their pitch wrong. Aerial measurement captures the real outline — which is why our cost estimates start from your actual footprint, not a rule of thumb.