Roof Tile and Slate Quantity Calculator
Estimate field tiles or slates from measured sloping roof area and compatible manufacturer-declared installed coverage. Add whole packs only when the actual units per pack are known.
How the estimate works
A is the supplied net actual sloping-face area in m². Extra area E = A × p / 100, where p is only the user-supplied extra percentage (blank means no extra is added). Adjusted area B = A + E. For installed density d in tiles/slates per m², unrounded quantity Q = B × d; for effective installed area c in m² per tile/slate, Q = B / c. Whole tiles/slates = ceil(Q). If actual units per pack n are supplied, whole packs = ceil(Q / n). Preserve full precision until these final ceilings; never round each face or an intermediate quantity.
Quantity estimate for field tiles/slates on an already-specified roof, not a setting-out, cutting or ordering plan. Use actual sloping-face area, not the footprint. Gross tile dimensions do not establish installed coverage or overlap/headlap. This tool does not choose or validate pitch, headlap, batten gauge, fixing, vents, accessories, structure, installation, permissions or compliance, and does not guarantee product suitability, stock, price or sufficient materials. It does not produce a Materials List.
Worked example
Source example, not a recommendation: Marley declares Modern installed covering capacity of 9.9 tiles/m² at 75 mm headlap. Roofing Outlet lists the Smooth Grey variant as a pallet of 192 tiles. Only if this matches your independently decided specification, 50 m² of net sloping area with an explicitly chosen 0% extra gives 50 × 9.9 = 495 tiles and ceil(495 / 192) = 3 pallets. Marley states a different rate at 100 mm headlap; do not reuse the 75 mm figure for another installation basis.
What to measure and check
Measure the actual sloping roof faces first, using the area sections or a reliable known net sloping area. Exclusions are already reflected in that area and are not deducted again. Then enter installed coverage from the exact chosen product and the independently decided roof specification. Combine faces only when the same installed coverage applies; calculate separately for different products or specifications.
Which roof area should I enter?
Use the actual net sloping faces, not the horizontal footprint. The area measurement tool can add named faces and their linked exclusions. Do not deduct openings again or apply another pitch factor to an already measured slope.
Can I use the dimensions printed on a tile or slate?
No. Gross length and width do not account for overlap, headlap or the installed covering pattern. Obtain effective installed tiles/slates per m² or m² per tile/slate from the exact product specification. Without compatible coverage this calculator reports the area only.
Will this choose headlap, pitch or battens?
No. Your roof specification must already be decided independently. The calculator does not determine headlap, pitch, gauge, battens, fixings, vents, ridges, hips, valleys, eaves or other accessories.
Why is the product specification and confirmation needed?
Installed coverage can change with the installation basis, even for the same product. Record the product and coverage specification and confirm that they match your already-decided roof. The calculator cannot independently verify the compatibility of those supplied facts.
How are packs and extra allowance calculated?
Extra is applied to area before product conversion. Only the final tile/slate requirement and final pack requirement are rounded up. Packs use the unrounded requirement divided by the exact supplied units per pack, not an assumed pallet size. Blank allowance adds no extra; blank pack count leaves packs unavailable.
Can several different roof faces share one calculation?
Yes, if their chosen product and installed coverage specification are identical: sum their net sloping areas before rounding. Where the product or coverage differs, calculate each compatible set separately. Area alone cannot determine edge cuts, reuse, special tiles or an order that is guaranteed sufficient.
Does this guarantee stock, price, suitability or compliance?
No. This is a coverage-based field-unit estimate, not installation or engineering advice, a stock or price check, a permission/compliance check, or a Materials List producer. Confirm the final specification and order with the responsible supplier or professional.
Where the numbers come from
Formula version v1.0. Implemented and tested 10 November 2026. Product-dependent figures remain your supplied facts; a cited example is not a selected product or a universal yield.
- Marley Modern concrete interlocking tile: installed covering capacity and specification — checked 2026-11-10
- Roofing Outlet: Marley Modern Smooth Grey, pallet of 192 tiles — checked 2026-11-10