A starting mix
A common starting mix for a raised bed is about 60% topsoil and 40% compost by volume, with drainage improved by the bed's open bottom — not by a gravel layer inside the bed, which can perch water. For heavy clay underneath, loosen the native soil before filling.
Settling
Fresh mix settles, often by 10–20% after watering. Fill to the top, water well, and top up before planting. Test your soil if you are unsure about pH or nutrients — a mix alone does not fix a nutrient problem. Use the soil volume calculator for quantities.