Soil Amendment Calculator
Enter your bed area and how deep you’ll till or top-dress to get the cubic yards, weight, and bag count of soil amendment you’ll need.
Measure straight across the widest point.
Adjust depth below if needed.
Covers spillage and compaction. 5–10% is normal.
Everything is calculated in your browser. Nothing you type is sent anywhere or stored.
Bags or bulk delivery?
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Coverage & weight
Material profile
For the default 15 ft × 10 ft bed tilled 3 inches deep, you’ll need about 1.53 cubic yards (0.76 tons) of compost blend, including 10% waste. Round up to 1 ton when ordering bulk — about $124.65 delivered, versus $581 in 0.5 cu ft bags.
Soil amendment needed for common project sizes
These estimates use a compost blend at 1,000 lb per cubic yard with a 10% waste allowance built in.
| Project | Depth | Cubic yards | Tons |
|---|---|---|---|
| Small bed mix-in, 100 sq ft | 3 in | 1.02 yd³ | 0.51 tons |
| Medium bed mix-in, 300 sq ft | 3 in | 3.06 yd³ | 1.53 tons |
| Surface top-dress, 500 sq ft | 1 in | 1.7 yd³ | 0.85 tons |
| Raised bed fill, 100 sq ft | 6 in | 2.04 yd³ | 1.02 tons |
| Small raised bed, 50 sq ft | 6 in | 1.02 yd³ | 0.51 tons |
| Large bed mix-in, 800 sq ft | 3 in | 8.15 yd³ | 4.07 tons |
Soil amendment types, weight and best use
| Type | Weight per yd³ | Best for | Drainage |
|---|---|---|---|
| Compost blend | 1,000 lb | General fertility boost | Good |
| Perlite / expanded mineral mix | 250 lb | Drainage & aeration | Excellent |
| Compost + soil conditioner blend | 900 lb | Clay-breaking, all-purpose | Good |
Figures are for dry material. Wet or freshly delivered material can weigh noticeably more.
How to calculate soil amendment you need
- Measure the bed area length × width in feet, or use the circle/border option for irregular beds.
- Set the amendment depth 1in for a surface top-dress, 3in tilled into an existing bed, 6in when filling a raised bed.
- Choose an amendment type compost for general fertility, perlite for drainage and aeration, or a compost/conditioner blend for breaking up clay.
- Add waste 10% covers spillage and settling as the amendment is tilled or worked in.
- Compare bulk vs. bagged cost bulk delivery is cheaper past roughly half a ton, especially for heavier compost-based blends.
Worked example
Using the calculator’s default 15 ft × 10 ft bed tilled 3 inches deep with compost blend:
- Area15 ft × 10 ft = 150 sq ft
- Depth in feet3 in ÷ 12 = 0.25 ft
- Volume150 sq ft × 0.25 ft = 37.5 cu ft
- Plus 10% waste37.5 × 1.10 = 41.25 cu ft
- Cubic yards41.25 ÷ 27 = 1.53 yd³
- Weight (compost blend, 1,000 lb/yd³)1.53 × 1,000 = 1,527.8 lb = 0.764 tons
- Order amountRound up to 1.0 ton — order in quarter-ton increments
- Bulk vs. bagged costBulk: 0.764 tons × $65/ton + $75 delivery = $124.65. Bags: 83 × $7.00 = $581.00
Recommended depth by project
| Project | Recommended depth | Notes |
|---|---|---|
| Surface top-dress | 1 in | Light annual maintenance layer worked into the top inch of soil |
| Standard bed mix-in | 3 in | Tilled into the top 6-8in for general fertility and structure improvement |
| Raised bed fill | 6 in | Blended with topsoil to fill or refresh a raised bed |
| Clay-breaking amendment | 3-4 in | Perlite or conditioner blend tilled deep to improve drainage in heavy clay |
| Container/pot mix | 10-20% by volume | Blended into potting soil rather than applied as a surface layer |
Bags or bulk delivery?
Bagged soil amendment (0.5 cu ft) is convenient for small beds and containers, but the per-ton cost is much higher than bulk delivery. Once a project needs more than about half a ton of material, bulk plus the flat delivery fee comes out ahead.
- Small beds/containersunder 0.5 ton (roughly 40 bags) — bags avoid a delivery minimum for a small job.
- Medium beds0.5 to 2 tons — compare the bag total against bulk price plus delivery.
- Large beds/plotsover 2 tons — bulk delivery is almost always cheaper and much less labor.
Common mistakes
- Tilling too shallowamendment worked into only the top inch or two doesn’t reach where most roots grow; till to at least 6-8 inches for annual beds.
- Using the wrong amendment for the problemcompost adds fertility and organic matter, but heavy clay needs a structural fix like perlite or coarse sand — compost alone won’t fix drainage.
- Skipping a soil test firstadding amendments blind can over-correct pH or nutrients that were already fine; a basic soil test tells you what’s actually needed.
- Applying amendment to dry soilamendments blend far more evenly into soil that’s lightly moist rather than bone dry or waterlogged.
- Not accounting for settlingloose, fluffy amendment settles noticeably after the first watering — this calculator’s waste factor helps, but expect some settling over the first few weeks.
- Over-amending sandy soil with perlitesandy soil already drains well; adding more drainage-focused amendment like perlite makes it drain even faster and dry out sooner — compost is usually the better fit there.
What this calculator assumes
Adjust your result if your project differs from these.
- Depth is the amount of amendment applied or tilled in, not the total bed depth.
- Default material is a compost blend at roughly 1,000 lb per cubic yard; perlite and other lightweight amendments weigh much less.
- 10% waste covers spillage and settling during tilling or spreading.
- Prices are material only — tilling equipment and labor are separate.
- Existing soil is assumed workable (not frozen or saturated) at the time of amending.
- Bag size is 0.5 cu ft, a common retail bag size for compost and soil amendment blends.
Common questions
How much soil amendment do I need per square foot?
For a standard 3-inch mix-in, figure about 0.25 cubic feet per sq ft before waste — or use the calculator above with your exact bed size.
What's the difference between compost and soil amendment?
Compost is one specific type of amendment focused on fertility and organic matter; “soil amendment” is a broader category that also includes perlite, sand, and conditioner blends aimed at fixing drainage or structure.
How deep should I till in soil amendment?
3 inches of amendment tilled into the top 6-8 inches of soil is standard for a new or refreshed garden bed.
Does perlite weigh the same as compost?
No — perlite is much lighter, roughly 250 lb per cubic yard versus about 1,000 lb per cubic yard for compost blends, since it’s an expanded volcanic mineral.
How much soil amendment for a raised bed?
A typical raised bed refresh uses about 6 inches of amendment blended with existing soil — for a 4×8 ft bed that’s roughly 1.2 cubic yards.
Can I over-amend my soil?
Yes — too much compost can over-fertilize and too much perlite can make soil dry out too fast. A soil test helps you amend based on what your soil actually needs.
How much does soil amendment cost for a small bed?
For the calculator’s default 150 sq ft bed at 3 inches, expect about $124.65 in bulk plus delivery versus $581 in bags.
When is the best time to amend soil?
Fall or early spring, a few weeks before planting, gives amendments time to integrate and settle before roots need them.
Do I need to amend soil every year?
Vegetable beds usually benefit from a fresh compost top-dress each year; structural amendments like perlite or sand are more of a one-time or occasional fix.
What amendment fixes clay soil best?
A blend of compost for organic matter and coarse material like perlite or expanded shale for structure works better than compost alone, which doesn’t solve drainage on its own.
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Project
| Shape | — |
| Dimensions | — |
| Depth | — |
| Area | — |
| Material | — |
| Waste allowance | — |
Volume & weight (incl. waste)
| Cubic yards | — |
| Cubic feet | — |
| Weight | — |
| Tons (rounded up) | — |
Shopping list
| Bulk order | — |
| Bagged alternative (0.5 cu ft) | — |
| Estimated cost | — |
Notes
Estimate generated by materialcalculate.com. Assumes uniform depth and standard dry material densities. Confirm quantities with your supplier before ordering.