How to use the backfill calculator
This calculator helps you calculate the net void remaining after a structure occupies an excavation. It is intended for quantity takeoffs, early budgets and checks against a measured schedule. Start with the dimensions or quantities for one clearly defined item of work. Enter values in the units shown beside each field; millimetres and metres are not interchangeable.
Outputs the geometric void to fill, not loose delivery volume. Compaction, suitability of excavated material, drainage zones and settlement require separate consideration.
Worked example
40 m³ excavated minus 12 m³ of buried structure leaves a 28 m³ net void. Do not apply a loose-volume factor without project-specific information.
Before you order
Compare the result with your drawings and supplier information. Keep the original net quantity separate from allowances, and round purchasing quantities only at the ordering stage. Check whether delivery, labour and related materials belong elsewhere in your estimate. If the drawing or specification changes, update the inputs and record the revised basis instead of reusing an earlier total.
For the concepts behind this calculation, read Takeoff (Quantity Takeoff), Measured Works, Unit Rate.