Enter the horizontal projection
Use the plan-view roof footprint including overhangs, not an area already measured along the slope. Rise and run must use the same length unit. A 6:12 pitch means six units of rise for twelve horizontal units. The calculator applies the geometric slope factor to the whole entered footprint.
For a simple roof where both sides have the same pitch, use the complete footprint once. Do not multiply by two just because there are two faces: both faces already project onto that footprint. For different pitches, calculate each horizontal section separately and add the resulting surface areas.
Turn area into a material request
The allowance covers your chosen provision for cuts and waste. It is not a roofing standard. Valleys, dormers, overlaps and the chosen product can change material quantities. A roofer or supplier should verify the takeoff; the tool does not assess structure, water drainage or suitability of a material for the pitch.
Use safe ground-based plans or professional measurements rather than climbing onto a roof. Transfer the measured surface area to the roofing budget calculator, taking care not to apply the same waste allowance twice.