An extra allowance is material purchased beyond the net area calculation. It can cover cuts, breakage, visual selection, later repairs, or other project-specific needs, but no single percentage is correct for every room and product. TileCalc leaves the field under your control. The useful habit is to confirm the net count first, choose a percentage from your plan and professional advice, and preserve that choice in the copied estimate.
Base the choice on known project conditions
Review the room shape, tile orientation, pattern, number of edges and penetrations, fragility, shade variation, availability, return policy, and the installer’s approach. These facts affect how much additional material may be useful, yet an area calculator cannot infer them from width and height. Ask the installer or supplier which allowance they expect and why. If advice differs, keep the net quantity visible and compare the resulting purchase counts rather than treating one percentage as a hidden fact. A simple grid with few cuts and a patterned layout are different purchasing decisions even when their measured area matches.
Understand where rounding changes the purchase
TileCalc applies the percentage to the unrounded area-based tile count, then rounds upward to a whole tile. If pieces per box is provided, it rounds again to a whole box, so the purchased piece count can exceed the requested percentage. That difference is packaging, not a second allowance. Record the selected percentage, pieces per box, box count, and final pieces. Before ordering, verify that every box belongs to the intended product and batch, confirm returns and future availability, and let the responsible professional make the final quantity decision. The estimate does not design a layout or promise that every cut will be reusable.