Trench Volume Calculator
The trench is modelled as a prism with a trapezoidal cross-section.
Enter Values
How to use this calculator
- Enter the trench length, bottom width and depth in metres.
- Set the side slope ratio if the walls are battered (horizontal run per 1 m of depth); leave it 0 or blank for vertical walls.
- Optionally add a spoil swell percentage to get the loose (bulked) volume you actually have to haul or stockpile.
How it works
The trench is modelled as a prism with a trapezoidal cross-section. The top width is the bottom width plus the batter on both sides: Wt = Wb + 2 × S × D, where S is the horizontal run per 1 m of depth. The cross-section area of that trapezoid is A = D × (Wb + S × D), the average of the top and bottom widths times the depth. Multiplying by the length gives the bank (in-ground) volume V = A × L.
With a slope ratio of 0 the walls are vertical and the formula collapses to the simple box A = Wb × D. Excavated soil bulks up, so the loose spoil volume is the bank volume multiplied by (1 + swell%/100). The approximate mass uses a typical soil density of about 1.8 t/m³ applied to the in-ground volume; adjust for your actual material.
Worked example
20 m battered trench for a pipe. A 20 m trench is dug 1.2 m deep with a 0.6 m bottom width and side slopes of 0.5 H : 1 V. Top width = 0.6 + 2 × 0.5 × 1.2 = 1.8 m. Cross-section area = 1.2 × (0.6 + 0.5 × 1.2) = 1.44 m². Bank volume = 1.44 × 20 = 28.8 m³. With 25% spoil swell the loose volume to haul is 28.8 × 1.25 = 36 m³, and the in-ground mass is about 28.8 × 1.8 = 51.84 t.
Common mistakes
- Confusing the side slope ratio with an angle — this field is the horizontal run per 1 metre of depth (e.g. 0.5 = half a metre out per metre down), not degrees.
- Using the top width as the bottom width. For battered trenches enter the narrower bottom width; the tool computes the wider top width for you.
- Comparing loose spoil volume against bank volume — loose (bulked) volume is always larger, so use the bank volume for the hole and the loose volume for haulage and stockpiles.
Frequently asked questions
What is the difference between bank volume and loose volume?
Bank volume is the compact, in-ground volume you excavate — that is the size of the hole. Once dug, soil bulks up (swells), so the same material takes more space as loose spoil. The loose volume, which is what you truck away or stockpile, equals the bank volume times (1 + swell%/100). Typical swell is around 15–30% for common soils.
How do I enter the side slope for a battered trench?
Enter the horizontal run for every 1 metre of vertical depth. A 1:1 slope is 1, a 2H:1V slope is 2, and a batter of half a metre out per metre down is 0.5. Use 0 (or leave it blank) for straight vertical walls.
Does this include any allowance for bedding, pipe or over-excavation?
No. It calculates the geometric volume of the trench prism only. Add your own allowances for pipe bedding, haunching, working room or over-dig, and confirm the soil density before converting volume to tonnes.
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