EXCAVATION & PAVING · FIELD TOOL

Trench Backfill Calculator

Trench backfill is calculated by zone: bedding (length × width × bedding depth), surround (length × width × (pipe OD + cover) − pipe volume) and general backfill above. A 50 m trench, 0.8 m wide, 1.5 m deep with a 300 mm pipe needs 6.0 m³ bedding, 20.5 m³ surround and 30.0 m³ backfill.

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Total backfill (compacted)

56.47m³

Bedding	6.00 m³
Pipe surround (pipe deducted)	20.47 m³
General backfill	30.00 m³
Granular to order (loose)	31.76 m³
Granular weight	52.9 t
Volume taken by pipe	3.53 m³

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Working

Bedding = L × W × 150 mm = 6.00
Surround = L × W × (300 + 300) − pipe (π/4 × OD² × L = 3.53) = 20.47
General backfill = L × W × (1,500 − 150 − 300 − 300) = 30.00
Granular loose = (6.00 + 20.47) × (1 + 20%) = 31.76; weight = compacted × density = 52.9 t

Results are estimates for planning and material ordering only. They are not a substitute for structural design or approval by a qualified engineer. Read the full disclaimer.

Formulas

Bedding     = L × W × bedding depth
Surround    = L × W × (OD + cover) − π/4 × OD² × L
Backfill    = L × W × (depth − bedding − OD − cover)
Granular    = (bedding + surround) × (1 + compaction %)   (loose to order)

Worked Example

A 50 m trench, 0.8 m wide, 1.5 m deep, 300 mm pipe, 150 mm bedding, 300 mm surround above the crown:

Bedding = 50 × 0.8 × 0.15 = 6.00 m³
Surround = 50 × 0.8 × 0.6 − 3.53 (pipe) = 20.47 m³
General backfill = 50 × 0.8 × 0.75 = 30.00 m³
Granular to order = 26.47 × 1.2 = 31.8 m³ loose (≈ 53 t at 2,000 kg/m³ compacted)

Per 10 m of Trench (0.8 m wide, 1.5 m deep, 150 mm bed, 300 mm cover)

Pipe OD	Bedding	Surround	Backfill	Granular (compacted)
110 mm	1.20 m³	3.18 m³	7.52 m³	4.38 m³
160 mm	1.20 m³	3.48 m³	7.12 m³	4.68 m³
225 mm	1.20 m³	3.80 m³	6.60 m³	5.00 m³
300 mm	1.20 m³	4.09 m³	6.00 m³	5.29 m³
450 mm	1.20 m³	4.41 m³	4.80 m³	5.61 m³

Frequently Asked Questions

How do I calculate trench backfill for a pipe?

Split the trench into zones. Bedding is length × width × bedding depth. The surround is length × width × (pipe OD + cover above the crown) minus the pipe's own volume. Everything above is general backfill.

How much granular bedding do I need?

Add the bedding and surround volumes (compacted), then add 15–25% for compaction when ordering by loose volume, or multiply the compacted volume by its compacted density to order by weight.

How deep should pipe bedding be?

Commonly 100–150 mm under the pipe, and the surround 150–300 mm above the crown. Follow the pipe manufacturer's and authority's specification.

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