Free Ceiling Area calculator with clear step-by-step results.
A flat ceiling matches the floor footprint, but a sloped or coffered one has more surface than the plan suggests - a 25 degree pitch adds about 10%. The waste allowance covers offcuts around light fittings and the paint figure assumes two coats at the coverage rate you enter.
Sloped surface
Surface = length x width / cos(pitch angle)
Paint required
Litres = surface x (1 + waste) x 2 coats / coverage per litre
No. Recessed fittings are small relative to the ceiling and you still need material around them, which is what the waste allowance covers.
The drops and returns of each recess create vertical surface that does not appear on plan. Eighteen percent is a working average; measure the reveals for an exact figure.