Trench Fill Calculator — Concrete Volume & Cost Comparison

Trench fill is a foundation method where the entire trench is filled with concrete to within 150mm of ground level. Unlike strip foundations (where only the base is concrete, with blockwork above), trench fill uses more concrete but eliminates the need for below-ground blockwork. It’s faster and requires less skilled labour. See our Foundation Calculator to compare trench fill with strip foundations.

Trench fill concrete volume = Total Length x Depth x Width. For example, a 30m trench at 1m deep and 600mm wide = 30 x 1.0 x 0.6 = 18 m³. Add 10% for over-dig and wastage = 19.8 m³. Use C25/30 (RC 25/30 or FND 2) concrete as standard per BS 8500-2:2023 — note GEN 4 is C20/25, so C25/30 moves into the designated RC/FND series. Use higher grades (RC 30/37 or FND 3/4 designations) for aggressive ground.

It depends on the balance between concrete and labour cost. Trench fill is heavy on ready-mix concrete (3–4× the volume of strip) but light on labour — there is no below-ground blockwork to lay. Strip is the opposite: less concrete, but a bricklayer must build the blockwork up to DPC. On deeper trenches (1m+) trench fill often wins overall because the saved bricklaying labour and avoided trench shuttering outweigh the extra concrete, and it is faster. Note: the cost comparison in this calculator is materials only (concrete + blocks) — it does not include labour, so its headline figure will normally favour strip. Treat it as a materials check, not a full like-for-like total cost.

The minimum trench width for trench fill is typically 450mm for single-leaf walls or 600mm for cavity walls. The width must be at least 150mm wider than the wall it supports. In practice, the trench width is often determined by the digger bucket size — 450mm or 600mm being standard mini-digger bucket widths.

Standard trench fill uses C25/30 (RC 25/30 or FND 2) concrete per BS 8500-2:2023 — note GEN 4 is C20/25, so C25/30 moves into the designated RC/FND series. In sulphate-bearing soils, sulphate-resistant concrete (DC-2 or higher) is required. In contaminated ground, use designated concrete to BS 8500-1. The building control officer or structural engineer will specify the required grade based on a soil analysis report.