Inputs
Example data table
| Scenario | Shape | Size | Thickness | Waste | Adjusted volume | Estimated weight |
|---|---|---|---|---|---|---|
| Raised bed base | Rectangle | 2.4 m x 1.2 m | 5 cm | 10% | 0.17 m³ | 0.27 t |
| Planter ring | Circle | 1.0 m diameter | 8 cm | 8% | 0.07 m³ | 0.11 t |
| Path trench | Custom area | 3.5 m² area | 10 cm | 12% | 0.41 m³ | 0.66 t |
Formula used
- Area: Rectangle = L x W, Circle = pi x (D / 2)², Custom = A
- Loose volume: Vloose = Area x Thickness
- Adjusted volume: Vadj = Vloose x Compaction x (1 + Waste%)
- Weight: W = Vadj x Density
- Drainage storage: S = Vadj x Porosity
- Cost: Material = Unit Price x Volume or Weight, Total = Material + Delivery
How to use this calculator
- Select your unit system, then choose a shape.
- Enter dimensions or an area, plus thickness.
- Set sections if repeating the same bed or trench.
- Adjust density, compaction, and waste for realism.
- Add pricing details to estimate material and total cost.
- Press Calculate to view results above the form.
- Use Download buttons to export CSV or PDF.
Why a gravel drainage layer matters
A well-sized gravel drainage layer reduces waterlogging around roots and limits anaerobic conditions in containers, raised beds, and planting pits. By creating void space, gravel temporarily stores water, then releases it to lower layers or drain outlets. It also helps protect soil structure during heavy irrigation and short storms. It can also reduce salt buildup by flushing excess irrigation from containers.
Choosing thickness for the site
Thickness depends on soil texture, plant tolerance, and how fast water must move. For many planters and beds, 3–8 cm is adequate. Use 10–15 cm when you expect intense rainfall, very fine backfill, or restricted outlets. Overbuilding can reduce root zone depth, so balance drainage with soil volume.
Understanding density, porosity, and compaction
Suppliers quote gravel by volume or weight, so density converts between them. Porosity estimates how much water the voids can hold; rounded stone often has higher void space than crushed mixes. Compaction accounts for settling after placement and vibration. Adding a modest waste allowance covers grade adjustments and handling loss.
Layout inputs and section planning
Rectangles suit beds and trenches, circles suit tree rings, and custom area suits irregular borders. When repeating the same footprint, sections multiply results without re-entry. Measure inside edges, not outside frames, and confirm thickness after leveling. If using geotextile, include overlap allowances in your area measurement.
Cost estimating and practical procurement
Use the pricing mode that matches your supplier: cubic meter, cubic yard, or metric ton. Include delivery fees because transport often dominates small projects. Order slightly above the adjusted volume to avoid delays, especially when matching stone size. Confirm stone gradation and cleanliness to prevent fines from clogging drainage paths. Buy washed gravel when possible to keep outlets clear and predictable long-term.
FAQs
Should I always add gravel under raised beds?
Not always. Many raised beds drain well with coarse compost and open bottoms. Add gravel when the base is sealed, outlets are limited, or you need a stable, free-draining base under a liner or container.
What gravel size works best for drainage?
Clean, angular or rounded stone around 10–20 mm typically drains well. Avoid mixes with many fines because they reduce void space and can clog outlets. Match size to your drain holes and geotextile.
How do I estimate porosity if I do not know it?
Use a typical range of 0.30 to 0.40 for many gravels. Rounded stone often trends higher, crushed blends lower. If you need precision, fill a bucket with gravel and measure water needed to saturate voids.
Does a drainage layer stop overwatering problems?
It helps move water away, but it cannot compensate for constant saturation. Improve irrigation scheduling, ensure outlets are clear, and use a suitable soil mix. Roots still need oxygen, even with a gravel layer.
Should I include compaction and waste allowances?
Yes. Gravel settles during placement, and small losses occur during handling. A compaction factor around 1.03–1.10 and waste around 5–15% usually covers leveling, edge trimming, and uneven subgrades.
Why do my cost estimates differ from supplier tickets?
Suppliers may bill by truck capacity, moisture, or rounded increments. Density can vary by gradation and quarry. Confirm their billing unit, minimum load, and whether delivery fees, taxes, or surcharges are included.