Calculator inputs
What this calculator does
This calculator estimates how much extra bed mix you should place above your target finished depth. Raised beds often settle after watering, microbial breakdown, and repeated planting cycles. If you start at the exact target depth, the finished surface can drop lower than planned.
The model combines a base settling rate with organic matter content, moisture behavior, repeated compression cycles, and a material profile multiplier. It then converts that combined rate into a practical allowance depth and an allowance volume. This helps gardeners buy enough material, avoid thin soil layers, and keep the final bed level closer to plan.
Plotly graph
The chart compares your target settled depth, required depth before tolerance, total allowance, recommended initial depth, and estimated post-settling depth before final leveling.
Example data table
| Scenario | Material | Area (m²) | Effective rate (%) | Recommended initial depth (cm) | Total allowance (cm) | Allowance volume (m³) |
|---|---|---|---|---|---|---|
| Leafy vegetable bed | Loam rich mix | 2.88 | 12.22 | 30.48 | 5.48 | 0.158 |
| Compost rich herb bed | Compost heavy mix | 3.60 | 23.75 | 42.34 | 12.34 | 0.444 |
| Woody raised bed | Woody raised bed blend | 1.62 | 16.79 | 44.56 | 9.56 | 0.155 |
Formula used
1. Organic factor = 1 + (organic matter share ÷ 200)
2. Cycle factor = 1 + ((settlement cycles - 1) × 0.04)
3. Effective settling rate = base rate × organic factor × moisture multiplier × material multiplier × cycle factor
4. Required depth before tolerance = desired settled depth ÷ (1 - effective settling rate)
5. Settlement depth = required depth before tolerance - desired settled depth
6. Total allowance = settlement depth + finish tolerance
7. Recommended initial depth = desired settled depth + total allowance
8. Initial fill volume = bed area × recommended initial depth
9. Allowance volume = bed area × total allowance
The finish tolerance is a practical leveling margin. It helps when the bed surface needs final raking after watering and planting.
How to use this calculator
- Enter the bed length and width in meters.
- Enter the soil depth you want after the bed has settled.
- Choose a base settling rate that fits your fill blend.
- Add the estimated organic matter percentage.
- Set a moisture multiplier. Wet blends usually settle more.
- Enter the number of expected compression cycles.
- Add a finish tolerance for final leveling and touch-up work.
- Select the nearest material profile.
- Press calculate and review depth, allowance, and volume outputs.
- Download the result as CSV or PDF if needed.
FAQs
1. What is bed settling allowance?
It is the extra fill depth added above your target soil depth. The allowance covers expected compression, organic breakdown, and final surface leveling after the bed settles.
2. Why does organic matter increase settling?
Compost, leaf mold, and woody particles shrink as they decompose and compact. Higher organic content usually means a larger loss of depth, so the calculator raises the allowance.
3. Should I use the same rate for every bed?
No. Fresh fills, compost-heavy blends, wet beds, and low-drainage mixes settle differently. Use a base rate that matches your actual material and adjust the multipliers carefully.
4. What does the moisture multiplier do?
It scales the settling rate to reflect wetter or drier conditions. Repeated wetting can collapse air pockets faster, especially in light, organic, or newly blended materials.
5. Why add a finish tolerance?
Finish tolerance gives you a small construction margin above the mathematical settlement value. It helps keep the finished surface even after raking, watering, and small material losses during planting.
6. Can I use this for in-ground beds?
Yes, but it works best for raised or clearly filled beds. Native soil layers usually settle less than imported mixes, so use a lower base settling rate when estimating in-ground changes.
7. What if my bed contains wood at the bottom?
Choose a woody material profile and use a realistic settling rate. Beds with buried branches or coarse fill often compress more as voids close and organic pieces break down.
8. Is the result exact?
No. It is a planning estimate based on practical gardening assumptions. Real settlement changes with weather, watering habits, particle size, age of compost, and how firmly the mix was placed.