Roof Tile Quantity Calculator

How to Use This Calculator

Start by measuring the horizontal projection of your roof, which is the footprint area you see from a top-down view (or the area inside the building's perimeter). If you have plans, you can read this area directly. Then determine the slope of the roof in degrees. If you only have the pitch percentage, convert it to degrees using the formula: degrees = arctan(pitch% / 100). For example, a 25% pitch gives approximately 14.04°. Next, measure a single tile. Measure its full length and full width in centimeters, including the area that will overlap with adjacent tiles. The overlap is crucial—using only the exposed part will overcount tiles because each tile covers less net area due to overlapping.

Enter these values and click 'Calculate'. The tool will correct the horizontal area for the slope using cosine, then divide by the effective coverage area of one tile in square meters. Finally, it adds 5% for breakage and wastage, rounding up to the nearest full tile.

Formula and Methodology

The underlying principle is that a tiled roof is a sloped plane, so the actual surface area is larger than the horizontal projection. The factor is 1/cos(slope). For example, for a 30° slope, that factor is about 1.155, meaning the actual area is 15.5% larger than the horizontal area. After calculating the corrected area, you divide by the area covered by each tile

Tile coverage area is measured as the length × width of the tile, but since tiles overlap, the 'covering' is less than the physical size. For example, if a tile is 40 cm × 25 cm, its area is 0.1 m². But with overlapping, each tile might only cover 0.09 m². For best accuracy, use the 'exposed surface' size or the manufacturer's 'tiles per m²' value. This calculator uses the full physical dimensions; you should adjust by measuring only the exposed part if that differs from full size. For beginners, a common approximation is to use a coverage area of 0.08–0.09 m² per tile for typical sizes. The calculator gives a ballpark; always confirm with the roofing supplier.

The waste factor accounts for cuts around edges, valleys, hips, and chimneys, as well as breakage during installation. For a simple gable roof, 5% is usually sufficient. For complex geometries, increase the waste manually by adding a few tiles to the result.

Practical Examples

Consider a house with a horizontal roof area of 120 m² and a slope of 25°. The cosine of 25° is 0.9063, so the effective roof area is 120 / 0.9063 = 132.4 m². Suppose you are using Portuguese tiles measuring 40 cm × 25 cm, which yields an area of 0.10 m² per tile. Without waste, you would need 132.4 / 0.10 = 1324 tiles. With 5% waste, you add 66.2 tiles, totaling approximately 1390 tiles. Since you can't buy partial tiles, you round up to 1390 or 1400 to be safe.

If the slope were 45° instead, the effective area would be 120 / cos(45°) = 170.7 m², needing 1707 tiles without waste, and about 1792 with waste. This demonstrates that a steeper roof significantly increases tile count due to the extra surface.

Tips and Best Practices

Always double-check the slope angle—an error of a few degrees can change the tile count substantially. Use a digital inclinometer or a smartphone app for accuracy. When measuring tile size, actually place two tiles on a flat surface and slide one to mimic the intended overlap. Measure from the edge of the lower tile to the interior edge of the upper tile to get the 'covering length'. This is more accurate than using nominal sizes.

Always order a few extra tiles (beyond the 5%) if your roof has complicated details. It's better to have leftovers than to run out mid-project. Also, check if the manufacturer offers a tiles per m² specification; if so, you can bypass the tile area calculation and simply multiply corrected area times that number. Finally, remember that waste is not only for breakage—it also accounts for cutting tiles along edges. If you have many penetrations (skylights, chimneys), add an extra 1-2%.