How Much Compost Do I Need for a Raised Bed?
Use our raised bed compost calculator to work out m³ and litres, then allow 10–25% extra for settlement.
Filling a raised bed starts with calculating the correct volume of compost or topsoil based on the internal dimensions of the bed. Because compost and soil naturally settle after installation, we recommend allowing 10–25% extra volume — especially when using compost-rich blends.
Quick Answer (Calculator Formula)
Length (m) × Width (m) × Depth (m) = Cubic Metres Required (m³)
Tip: Once you’ve calculated your base volume, add 10–25% extra to allow for natural settlement and compaction in raised beds.
Worked Example
- Length: 2m
- Width: 1m
- Depth: 0.4m (400mm)
2 × 1 × 0.4 = 0.8m³Allow 10–25% extra: 0.88–1.00m³ (round up to suit available pack sizes).
Common Raised Bed Sizes & Compost Requirements
| Raised Bed Size | Depth | Base Volume | Recommended Order (10–25% extra) |
|---|---|---|---|
| 1m × 1m | 300mm | 0.30m³ | 0.33–0.38m³ |
| 2m × 1m | 300mm | 0.60m³ | 0.66–0.75m³ |
| 2m × 1m | 450mm | 0.90m³ | 0.99–1.13m³ |
| 3m × 1.2m | 400mm | 1.44m³ | 1.58–1.80m³ |
Note: These figures are estimates. Compost-rich (high organic matter) blends may settle more after installation — lean towards the 25% end when levels matter.
Why Raised Beds Need Extra Compost
Raised beds and containers often need a little extra because the material continues to settle after installation due to:
- Watering and rainfall
- Gravity and natural consolidation
- Reduction of air spaces
- Root activity
- Natural decomposition (especially in compost blends)
Compost vs Topsoil — Which Settles More?
- Compost-rich blends: typically settle more (higher organic matter)
- Enriched topsoil: moderate settlement
- Screened mineral topsoil: usually least settlement
Organic materials compress more easily than mineral soil particles and can reduce in volume as they settle and break down over time.
Recommended Depth for Raised Beds
- 150–300mm — herbs and shallow roots
- 300–450mm — vegetables and general planting
- 450mm+ — deep-rooting crops
Frequently Asked Questions
- How do I calculate compost volume for a raised bed?
Multiply the internal length × width × depth in metres to get cubic metres (m³). Then add 10–25% extra to account for settlement in raised beds.
- Why does compost settle after I fill the bed?
Compost settles due to watering, gravity, reduction of air spaces and natural decomposition of organic matter. Compost-rich blends often settle more than mineral topsoil.
- How much extra should I allow for raised beds?
We recommend 10–25% extra. Use 10–15% for lower organic matter topsoil, and closer to 25% for compost-rich blends, deep beds, and where you want the finished level to stay high.
- Do you sell different pack sizes?
Yes — this calculator shows guidance for 1m³ bulk bags (1000L), 1200L jumbo bags, and 50L and 60L bags.
Need Help Calculating?
If you’re unsure how much compost or topsoil to order for your raised bed project, our team can help. Contact Quality Garden Supplies for friendly advice.