Inputs
Example data table
| Scenario | Inputs | Outputs |
|---|---|---|
| French drain trench |
Length 12 m, width 0.30 m, depth 0.45 m Pipe Ø 100 mm, waste 10%, density 1600 kg/m³ |
Gravel ≈ 1.78 m³ (≈ 2.33 yd³) Weight ≈ 2,848 kg (≈ 142 bags of 20 kg) |
| Planter layer |
Bed 3 m × 1.2 m, thickness 8 cm Waste 8%, density 1500 kg/m³ |
Gravel ≈ 0.31 m³ (≈ 0.41 yd³) Weight ≈ 459 kg (≈ 23 bags of 20 kg) |
| Cylindrical pit |
Diameter 40 cm, depth 60 cm Waste 5%, density 1600 kg/m³ |
Gravel ≈ 0.08 m³ (≈ 0.11 yd³) Weight ≈ 132 kg (≈ 7 bags of 20 kg) |
Formula used
- Rectangular trench volume: V = L × W × D
- Planter layer volume: V = L × W × T
- Cylindrical pit volume: V = π × (d/2)² × h
- Pipe subtraction (optional): Vpipe = π × (d/2)² × L
- Waste/overfill: Vfinal = (V − Vpipe) × (1 + waste%/100)
- Weight estimate: W = Vfinal × bulk density
How to use this calculator
- Select the shape that matches your drainage space.
- Enter dimensions and pick the correct units for each field.
- Set a waste percentage to cover settling and spillage.
- If a perforated pipe is inside, enter its diameter and length.
- Enter density for a better weight estimate and bag planning.
- Click calculate to see results above the form, then export.
Drainage performance and gravel grading
A drainage layer works when void space stays open. Clean, angular gravel (often 10–20 mm) resists fines migration and maintains flow paths. Pair it with filter fabric where soils are silty to reduce clogging and preserve long-term discharge.
Converting trench geometry into volume
This tool converts your selected shape into a consistent meter-based volume. Rectangular trenches use length × width × depth, planter layers use bed area × thickness, and cylindrical pits use π × radius² × depth. Optional pipe subtraction removes the pipe cylinder from the total.
Waste allowances and compaction behavior
Ordering exactly the computed volume is risky. Spillage, uneven excavation, and settlement after watering can reduce the final level. Many landscaping crews apply 5–15% waste/overfill; tighter sites may use 5–8%, while longer runs with hand placement often use 10–15%.
Weight, transport, and ordering strategy
Bulk density varies by stone type and moisture. Using 1,450–1,700 kg/m³ gives a practical weight estimate for delivery planning. The calculator converts weight into bag counts and approximate truckloads so you can compare bagged versus bulk pricing and ensure access for equipment.
Field example and checks
Example data: a 12 m trench, 0.30 m wide, 0.45 m deep with a 100 mm pipe and 10% allowance yields about 1.78 m³ (2.33 yd³). At 1,600 kg/m³, order roughly 2,850 kg—about 142 bags of 20 kg. Sanity check by confirming your excavated width and depth at several points and re-running the calculation if the trench varies.
FAQs
1) What gravel size is best for a French drain?
Clean, washed gravel around 10–20 mm is commonly used. It keeps voids open, drains well, and surrounds perforated pipe without quickly clogging when paired with fabric in fine soils.
2) Should I always subtract the pipe volume?
Subtracting pipe volume slightly improves accuracy, especially for narrow trenches. For wide trenches, the difference is small, but it can still help when ordering close to minimum delivery quantities.
3) How much waste percentage should I use?
Use 5–8% for controlled installs with consistent trenching. Use 10–15% when excavation is irregular, gravel is hand-placed, or you expect noticeable settlement after watering and backfilling.
4) What density should I enter for weight estimates?
If you do not have supplier data, start with 1,600 kg/m³. Washed stone can be lower and wetter material can be higher. Update the value using your supplier’s bulk density for better ordering accuracy.
5) Can I use this for planter drainage layers?
Yes. Select the planter layer option, enter bed dimensions and the gravel thickness. Add waste to cover leveling and minor grade changes across the bed.
6) Why does my delivered volume look different than expected?
Volume can change with moisture, compaction, and how the load is heaped. Also confirm units and measured trench sizes. Recheck multiple points because trenches are rarely uniform end-to-end.
7) How do I decide between bagged and bulk gravel?
Bagged gravel suits small jobs and tight access. Bulk loads reduce unit cost for larger volumes but require delivery clearance. Compare the calculator’s bag count and truckload estimate to supplier pricing.