Formula
Plan area = plan length × plan width. Slope factor = 1 ÷ cos(pitch angle). Sloped roof area = plan area × slope factor. Order area = sloped roof area × (1 + waste percent ÷ 100). Packs = ceiling(order area ÷ coverage per pack).
Trade & Construction
Roofing Calculator
Calculate roof area, pitch-adjusted slope factor, waste allowance and rounded roofing packs for a printable homeowner, supplier or tradie quote note.
Trade & Construction
Roofing Calculator
Live answerReadyCalculator queued
Live resultReadyCalculator queued
Formula used
Plan area = plan length × plan width. Slope factor = 1 ÷ cos(pitch angle). Sloped roof area = plan area × slope factor. Order area = sloped roof area × (1 + waste percent ÷ 100). Packs = ceiling(order area ÷ coverage per pack).
This is the method behind the answer, so the result can be checked rather than simply trusted.What-if check
Waste and pitch sensitivity
Compare the same roof plan with different waste allowances and nearby pitch angles before turning the estimate into a supplier order.
| Waste | Order area | Rounded packs |
|---|---|---|
| 0.0% | 66.20 m² | 23 packs |
| 5.0% | 69.51 m² | 24 packs |
| 10.0% | 72.82 m² | 25 packs |
| 15.0% | 76.13 m² | 26 packs |
| Pitch | Slope factor | Order area |
|---|---|---|
| 20.0° | 1.0642 | 70.24 m² |
| 25.0° | 1.1034 | 72.82 m² |
| 30.0° | 1.1547 | 76.21 m² |
Visual proof
Plan area × slope factor
The sketch shows the core geometry: the flat roof plan is multiplied by a pitch slope factor, then waste and pack rounding are added separately.
Visual grid
This result measures part of the space you live in
Length, area, volume and material estimates are grid problems too: measure the space, account for edges and allowances, then turn the pattern into a number you can use.
Micro-timehours, minutes, shiftsHuman scaledays, weeks, projectsMacro-timemonths, years, calendars
Measured outputReady
Space calculations turn a real surface, room, run or volume into cells, edges and allowances that can be quoted, ordered or checked.
CalculationTime
Roofing Calculation Report
ReadyCalculator queued
Inputs
- Plan length
- 10 m
- Plan width
- 6 m
- Roof pitch
- 25 degrees
- Waste allowance
- 10 %
- Coverage per pack
- 3 m²
Method
Plan area = plan length × plan width. Slope factor = 1 ÷ cos(pitch angle). Sloped roof area = plan area × slope factor. Order area = sloped roof area × (1 + waste percent ÷ 100). Packs = ceiling(order area ÷ coverage per pack).
- Plan area is 10 × 6 = 60 m². At 25°, slope factor is 1 ÷ cos(25°) = 1.1034, so sloped roof area is 66.20 m². Add 10% waste to get 72.82 m². At 3 m² per pack, 72.82 ÷ 3 = 24.27, so round up to 25 packs.
Assumptions
- The roof is treated as a simple rectangular roof plan with one pitch angle across the whole area.
- Pitch is entered in degrees and converted with the cosine slope factor.
- Coverage per pack is user-entered because shingles, tiles, metal sheets, membranes and rolls cover different areas.
- The result is a material-planning estimate only. It does not replace site measurement, structural design, fixings, flashing, underlay, safety access or local building-code checks.
Notes
Use this space on the printed report for client, supplier, classroom, job-location, measurement, quote or approval notes.
Worked example
Plan area is 10 × 6 = 60 m². At 25°, slope factor is 1 ÷ cos(25°) = 1.1034, so sloped roof area is 66.20 m². Add 10% waste to get 72.82 m². At 3 m² per pack, 72.82 ÷ 3 = 24.27, so round up to 25 packs.
Professional note
Master’s Tip: measure hips, valleys, dormers, penetrations and starter/ridge pieces separately. A clean rectangle is useful for the first quote note, but real roofs often need extra line items before ordering.
Regional and unit assumptions
Standard or basis: transparent geometry using rectangular plan area, degree pitch and cosine slope factor. No roofing-system warranty, wind rating, structural approval or local code compliance is claimed.
Assumptions and limitations
- The roof is treated as a simple rectangular roof plan with one pitch angle across the whole area.
- Pitch is entered in degrees and converted with the cosine slope factor.
- Coverage per pack is user-entered because shingles, tiles, metal sheets, membranes and rolls cover different areas.
- The result is a material-planning estimate only. It does not replace site measurement, structural design, fixings, flashing, underlay, safety access or local building-code checks.
Methodology & Accuracy
How this calculator is checked
CalculationTime pages are built around visible arithmetic: the formula, assumptions, worked example and practical limitations are shown so the result can be checked rather than simply trusted.
Formula used
Plan area = plan length × plan width. Slope factor = 1 ÷ cos(pitch angle). Sloped roof area = plan area × slope factor. Order area = sloped roof area × (1 + waste percent ÷ 100). Packs = ceiling(order area ÷ coverage per pack).
Standard or basis
Standard or basis: transparent geometry using rectangular plan area, degree pitch and cosine slope factor. No roofing-system warranty, wind rating, structural approval or local code compliance is claimed.
Where a calculator follows a named legal, trade or industry standard, that standard is cited visibly. Otherwise the page uses transparent general arithmetic and states its limits.Master's Tip
Master’s Tip: measure hips, valleys, dormers, penetrations and starter/ridge pieces separately. A clean rectangle is useful for the first quote note, but real roofs often need extra line items before ordering.
Related calculators
Questions
How do I calculate roof area from pitch?
Multiply the horizontal plan area by 1 divided by the cosine of the roof pitch angle. That converts flat plan area into sloped roof surface area.
Why is roof area larger than plan area?
A pitched roof is sloped, so the surface running up the roof is longer than the horizontal plan distance. The steeper the pitch, the larger the slope factor.
How much waste should I allow for roofing?
Simple rectangular roofs may use a lower allowance, while hips, valleys, cut sheets, pattern matching and complex details need more. This calculator makes the waste percentage visible so it can be adjusted before ordering.
Can this estimate shingles, tiles or metal sheets?
Yes, if you enter the correct coverage per bundle, tile pack, sheet set or roll. The calculator estimates area and rounded packs; it does not design the roof system.
Does this include ridge caps, flashing or underlay?
No. Those are separate items. Use the printed report as a roof-area record, then add accessories, fasteners, flashing, underlay and safety requirements from the supplier or installer.
Calculation note
Roofing quantity estimates start with geometry but become trade records when pitch, waste, pack coverage and roof details are written down. The calculator keeps the first area calculation visible so a quote or supplier conversation has a clear starting point.
Pitch turns plan area into roof surface
A roof plan seen from above is a flat rectangle, but the roofing material sits on a slope. The cosine slope factor adjusts the horizontal area into the longer sloped surface area. That is why steep roofs need more material than the same building footprint with a low pitch.
Waste depends on roof complexity
Straight rectangular runs are easier to cover than roofs with hips, valleys, dormers, penetrations and angled cuts. Keeping the waste percentage visible helps a homeowner, estimator or installer discuss the basis before material is ordered.
Area is only one part of a roof order
Roofing systems also need underlay, battens or deck checks, flashing, ridge and hip pieces, fasteners, ventilation and safety planning. This page estimates material coverage area; it deliberately avoids claiming structural or code approval.
Sources and further readingNRCA: Roofing Manual overviewThis Old House: Roofing materials and installation planning