A round hole is a cylinder; a round post inside it leaves an annular cylinder. ConcreteCalc uses the outer diameter, depth, and quantity you enter. When you also enter a smaller round inner or post diameter, it subtracts that circular cross-section. It does not estimate square posts, irregular excavation, bell-shaped bases, disturbed soil, or dimensions required for a load. Those are project decisions outside a volume calculator.
Measure diameters, not radii
Enter the full outer width across the circular opening and the vertical depth or height. The formula divides each diameter by two internally. Use quantity only for holes sharing all measurements. If a round post occupies the centre, enter its measured diameter as the inner value; it must be smaller than the hole. Leave inner diameter at zero when nothing is subtracted. A square post does not have an equivalent diameter supplied by ConcreteCalc, because inventing one would hide a different geometry.
Keep the π approximation deterministic
The volume formula is π × (outer radius² − inner radius²) × depth × quantity. ConcreteCalc represents π by the fixed rational value 3141592653589793 / 1000000000000000. That approximation avoids relying on a device-specific floating-point path and remains documented in the result. For an empty cylinder, inner radius is zero. For an annulus, subtract squared radii before multiplying by depth. Preserve the unrounded rational result through extra and bag calculations, then round only the displayed conversions.
Separate measured displacement from site uncertainty
Post displacement is a geometric subtraction, not an allowance. It reduces base volume because the entered round object occupies space. A chosen extra percentage is a separate addition applied afterward. Do not use one to disguise the other. Real holes can taper, crumble, fill with water, or differ from their planned diameter; the calculator cannot inspect those conditions. Measure each distinct group, keep any extra explicit, and confirm construction and purchasing decisions with the appropriate qualified people.