UnitConv

Roofing Calculator

Estimate roof area, squares and shingle bundles from the footprint and pitch

Roof area to buy (with waste)
115.95
m²
Pitch multiplier
1.0541
×
Roof area (no waste)
105.41
m²
Squares
12.48
squares (100 ft² each)
38bundles (3 per square)
≈ 7 rolls of underlayment
Roof area115.95 m²

Bar scaled to a 300 m² (≈3200 ft²) reference.

Formula

Multiplier = √( 1 + (pitch / 12)² )

Roof area = footprint × multiplier × (1 + waste% / 100)

Squares = roof area / 100 ft²

Bundles = ceil( squares × bundles per square )

The footprint is multiplied by the pitch factor to get the sloped roof area, then a waste allowance is added. Roofing is sold in squares (100 ft²) and shingles in bundles, usually 3 per square.

What is a Roofing Calculator?

A roofing calculator works out the real, sloped roof area and the materials to cover it from the building footprint and the roof pitch. Enter the footprint area, or its length and width, and the pitch as the rise per 12 of horizontal run. The tool turns the pitch into a multiplier, √(1 + (pitch/12)²), and multiplies the footprint by it to get the actual sloped surface, then adds a waste allowance. Roofing is measured in squares (one square = 100 ft²), and asphalt shingles are sold in bundles, usually 3 bundles per square. The calculator converts the roof area into squares and rounds the bundles up. Both metric (m) and imperial (ft) units are supported, for gable, hip and lean-to roofs.

How to Use

1. Choose metric or imperial units. 2. Enter the footprint area, or switch to dimensions and enter length and width. 3. Enter the roof pitch as the rise per 12 of run (e.g. 4 for 4-in-12). 4. Set a waste percentage (10% is typical; more for hips and valleys). 5. Read the sloped roof area, squares and shingle bundles.

Formula & Definition

Multiplier = √( 1 + (pitch / 12)² ) Roof area = footprint × multiplier × (1 + waste% / 100) Squares = roof area / 100 ft² Bundles = ceil( squares × bundles per square ) For example, a 100 m² footprint at a 4-in-12 pitch has a multiplier of √(1 + (4/12)²) ≈ 1.054, so the sloped area is about 105 m² before waste.

Interpreting Results

The headline figure is the sloped roof area you need to cover, including the waste allowance - always larger than the flat footprint because a pitched roof has more surface than the ground it sits on. The pitch multiplier shows how much the slope adds (1.054 at 4-in-12, 1.25 at 9-in-12). Squares are the trade unit: one square covers 100 ft², and shingles come in bundles, usually 3 per square, rounded up to whole bundles. Increase the waste percentage for roofs with many hips, valleys, dormers and penetrations. Order starter strips, ridge caps, underlayment and flashing separately; the optional underlayment estimate assumes standard roll coverage.

Frequently Asked Questions

How do I calculate roof area? ▾

Multiply the footprint by the pitch factor √(1 + (pitch/12)²) to get the sloped area, then add a waste allowance. For example, a 100 m² footprint at 4-in-12 is about 105 m². This calculator does it for you.

What is a roofing square? ▾

A square is the roofing trade unit equal to 100 square feet (about 9.29 m²) of roof area. Materials and labour are usually quoted per square, so converting your roof area into squares makes ordering easier.

How many bundles of shingles per square? ▾

Most architectural and three-tab asphalt shingles come 3 bundles to the square. Heavier or specialty shingles can be 4 or 5 bundles per square - check the product before ordering.

How much waste should I add for roofing? ▾

Around 10% for a simple gable roof. Add 15% or more for hip roofs and any roof with lots of valleys, dormers, chimneys and other cut-ins where off-cuts pile up.

What does roof pitch mean? ▾

Pitch is the steepness, written as the rise per 12 units of horizontal run - so 4-in-12 rises 4 for every 12 across. A steeper pitch means a larger sloped area and more material for the same footprint.

This tool provides general estimates for planning only. It does not include starter strips, ridge caps, flashing or hip and valley waste in detail; measure carefully and confirm quantities with your supplier before purchasing.