A 4×8 ft raised garden bed that is 12 inches (1 ft) deep needs 32 cubic feet of topsoil, or about 1.2 cubic yards. A 6-inch-deep bed needs 16 cu ft (0.6 cu yd), and an 18-inch-deep bed needs 48 cu ft (1.8 cu yd).
Instead of doing the math by hand, use the free Topsoil Calculator to size your bed in seconds — it works in cubic feet, cubic yards, and cubic meters.
Worked Example: 4×8 Bed at 12 Inches Deep
Here is the exact math, step by step:
- Convert depth to feet. 12 inches = 1 ft.
- Calculate volume in cubic feet. Length × width × depth = 4 ft × 8 ft × 1 ft = 32 cu ft.
- Convert to cubic yards. There are 27 cubic feet in a cubic yard, so 32 ÷ 27 = 1.185 → ≈ 1.2 cubic yards.
Different depths for the same 4×8 bed:
- 6 inches deep (0.5 ft): 4 × 8 × 0.5 = 16 cu ft → 16 ÷ 27 = 0.6 cu yd.
- 18 inches deep (1.5 ft): 4 × 8 × 1.5 = 48 cu ft → 48 ÷ 27 = 1.8 cu yd.
Metric version: A 1.2 × 2.4 m bed at 30 cm deep: 1.2 × 2.4 × 0.3 = 0.864 m³ ≈ 0.9 m³ of topsoil.
Quick Reference Table: Common 4×8 Bed Depths
Bookmark this table so you don’t have to redo the math:
| Depth | Cubic feet | Cubic yards | Cubic meters | Order (with settling) |
|---|---|---|---|---|
| 6 in (15 cm) | 16 cu ft | 0.6 cu yd | 0.45 m³ | 0.75 cu yd |
| 8 in (20 cm) | 21.3 cu ft | 0.8 cu yd | 0.60 m³ | 1.0 cu yd |
| 10 in (25 cm) | 26.7 cu ft | 1.0 cu yd | 0.76 m³ | 1.2 cu yd |
| 12 in (30 cm) | 32 cu ft | 1.2 cu yd | 0.90 m³ | 1.4–1.5 cu yd |
| 18 in (45 cm) | 48 cu ft | 1.8 cu yd | 1.36 m³ | 2.0–2.2 cu yd |
| 24 in (60 cm) | 64 cu ft | 2.4 cu yd | 1.81 m³ | 2.7–2.9 cu yd |
Note: metric equivalents assume a 1.2 × 2.4 m bed; “order” figures include a 10–20% settling allowance, rounded to what suppliers actually sell.
How to Measure Your Bed Correctly
The math only works if your measurements are right:
- Measure the inside, not the outside. A 4×8 bed built from 2×8 lumber has interior dimensions of roughly 3.7 × 7.7 ft once you account for board thickness at both ends. Using the outside dims overstates volume slightly — fine for ordering, but know it’s an overestimate.
- Measure depth from the soil surface you’ll actually fill to. If you plan to leave 2 inches of headroom below the rim, your filled depth is 10 inches, not 12 — that shaves about 5.3 cu ft off the total.
- Check for slope. On a sloped yard, one side of the bed may be 12 inches deep while the other is only 9. Average the depths, or fill the low side with a few inches of gravel or coarse material first.
- Account for existing soil. If you’re topping up an existing bed rather than filling from scratch, measure the gap between the current soil line and your target fill line — you may need far less than a full bed’s volume.
What Changes the Amount
- Bed depth. This is the biggest variable — volume scales linearly with depth, so doubling depth doubles the soil needed.
- Settling. Fresh topsoil settles 10–20% in the first weeks of watering and rain. For a 12-inch bed, order about 1.4 cubic yards instead of exactly 1.2 to end up with a full bed after settling.
- Bottom fillers. Many gardeners fill the bottom of deep beds with logs, sticks, or coarse organic matter (hugelkultur-style). Every 4 inches of filler in a 4×8 bed saves about 10.7 cu ft of topsoil.
- Compaction. Soil you walk on or tamp down takes up less volume than loose soil — compacted fills need extra material to reach the same finished level.
Common Mistakes
- Ordering the exact volume with no settling allowance. A bed filled to 1.185 cu yd of fresh soil will sit an inch or two low within a month. Add 10–20% extra.
- Confusing cubic feet with cubic yards. 32 cu ft is only 1.2 cu yd — ordering “32 yards” would dump 864 cu ft of soil on your driveway. Always check the unit on your order.
- Ignoring internal dimensions. Lumber thickness eats space: a bed built from 2×8 boards has interior dimensions closer to 3.75 × 7.75 ft, slightly reducing the volume. The difference is small (about 1 cu ft less), but worth noting for precision.
FAQ
How much topsoil for a 4×8 raised bed 12 inches deep, exactly?
Exactly 32 cubic feet, which converts to 1.185 cubic yards — round to 1.2 cubic yards. Plan to order about 1.4 cubic yards to allow for settling.
Can I use the Topsoil Calculator for other bed sizes?
Yes — enter any length, width, and depth, and the Topsoil Calculator returns the volume in cubic feet, cubic yards, or cubic meters.
Is 12 inches of soil deep enough for a raised garden bed?
Yes for most vegetables, herbs, and flowers — 10–12 inches suits the vast majority of plants, while root crops like carrots and parsnips appreciate 14–18 inches.
For the full breakdown of raised bed sizing, delivery options, and costs, run your numbers on the Topsoil Calculator, then read the complete guide and the cost guide.