How to Calculate Excavation & Backfill: Volumes, Swell & Compaction (2026 Guide)
Calculating excavation and backfill accurately determines haulage quantities, the number of truck trips, and site work cost, and prevents surprises of excess excavated soil or a backfill material shortage. In this guide you will learn step by step: how to calculate excavation volume for foundations, the difference between bank and loose volume, the swell (bulking) factor, the backfill quantity by soil type and compaction factor, and the safe excavation side slope per the Saudi code. All numbers here match exactly the free Excavation and Backfill calculators on “Site Engineer”.
🧮 The Excavation & Backfill Formula
- Excavated (loose) soil = bank volume × (1 + swell %); default swell 25%.
- Truck trips = excavated soil ÷ truck capacity (default 14 m³), rounded up.
- Backfill: material needed = compacted fill volume × the material's fill factor (sand 1.20 … clay 1.40).
⚠️ Key distinction: bank volume = soil in the ground, loose volume = after excavation (increases by swell). For sloped sides the calculator uses a trapezoidal average (volume grows with slope). The fill factor is above 1 because the material compacts when rammed, so more loose material is needed.
📋 Steps to Calculate Excavation & Backfill
- Calculate excavation (bank) volume: Multiply length × width × depth for each footing or trench, and sum all similar excavations.
- Add side slope: For sloped sides choose the slope ratio (1:2 or 1:1); the calculator uses a trapezoidal average for safety.
- Calculate excavated soil: Bank volume × (1 + swell %). Default swell is 25% and varies by soil type.
- Calculate truck trips: Divide the excavated soil by the truck capacity (14 m³) and round up.
- Calculate backfill quantity: Compacted fill volume × the material fill factor: sand 1.20, gravel 1.15, common soil 1.30, clay 1.40.
- Spread and compact the fill: Spread the fill in layers (20–30 cm) and compact each layer to the required degree (90–95% Proctor).
✅ Worked Example
Excavation: a footing 4m × 3m × 2m deep, vertical sides, SBC code, 25% swell, 14m³ truck:
Enter the same values in the Excavation Calculator (and the Backfill Calculator for filling) and you'll get the exact same result — these values come from both calculators' formulas.
📊 Backfill & Compaction Factors by Material
| Material | Fill factor | Bulking % | Density (t/m³) | Compaction % |
|---|---|---|---|---|
| Sand | 1.20 | 20% | 1.7 | 95 |
| Gravel | 1.15 | 15% | 1.9 | 95 |
| Common soil | 1.30 | 30% | 1.6 | 90 |
| Fill soil | 1.25 | 25% | 1.5 | 90 |
| Clay | 1.40 | 40% | 1.8 | 85 |
🛡️ The Saudi Angle: Excavation Slope & Code
The Saudi SBC 303 code sets a safe side-slope angle for excavations depending on the soil type; in the calculator the approximate safe angle is: SBC 45° · BS 45° · ECP 40° · AASHTO 34°. Available slope ratios: vertical (0), 1:2, 1:1, 1:1.5 — and the deeper the excavation or the weaker the soil, the more slope or shoring is needed.
Tips: plan for site balance (cut = fill) to reduce haulage; keep part of the suitable soil for re-filling; never backfill over debris or organic soil. Spread the fill in even layers and compact each before the next.
🚫 Common Excavation & Backfill Mistakes
- Confusing bank with loose volume: bank is in the ground, loose is after excavation (+swell) — use loose for haulage and trucks.
- Forgetting swell (bulking): excavated soil is 15–40% larger than the excavation volume depending on the soil.
- Ignoring excavation side slope: deep vertical sides (> 1.5m) are prone to collapse — use a slope or shoring.
- Forgetting the fill factor: you need more material than the fill volume (sand +20%, clay +40%) because it compacts.
- Confusing compaction degree with the quantity factor: compaction degree (90–95% Proctor) is a quality measure, not the material quantity factor.