Last updated: June 19, 2026
Compost Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
The Compost Calculator finds how much compost to buy or make to cover a garden, raised bed, or lawn at a chosen depth. The formula is Volume (ft³) = Length (ft) × Width (ft) × Depth (in) ÷ 12, with cubic yards = ft³ ÷ 27 and bags (1 ft³ each) rounded up. For example, a 20 × 10 ft bed at 2 in needs 33.3 ft³ ≈ 1.23 yd³, or 34 bags. Typical depths are about 2 in for a new garden, 0.25–0.5 in for top-dressing a lawn, and 2–3 in for a vegetable bed. Always order ~10% extra to allow for 20–30% settling.
To calculate compost, multiply the length and width of the area in feet by the depth in inches and divide by twelve to get cubic feet. Divide cubic feet by twenty-seven for cubic yards. For example, a 20 by 10 foot bed at 2 inches deep needs 33.3 cubic feet, or about 1.23 cubic yards.
Key Takeaways
- Volume (ft³) = Length (ft) × Width (ft) × Depth (in) ÷ 12 — divide by 27 for cubic yards
- 1 cubic yard = 27 ft³ ≈ 13.5 standard 2 ft³ bags, and covers ~162 ft² at 2 inches deep
- Top-dressing a lawn: ~0.25–0.5 in; new garden bed: 1–2 in; vegetable garden: 1.5–3 in
- Order ~10% extra to allow for settling (20–30% in the first weeks) and uneven spreading
- Bulk delivery beats bags above ~1 cubic yard — typically 50–70% cheaper per yard
- More isn't better: > 4 in per year can build up phosphorus and salts
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
Volume (ft³) = Area (ft²) × Depth (in) ÷ 12
Where:
- V=Compost volume(ft³)
- A=Area to cover(ft²)
- d=Desired depth(inches)
Worked Examples
20×10 ft garden bed at 2 in
A typical home vegetable garden being top-dressed with a standard 2-inch compost layer.
- 1Area = 20 × 10 = 200 ft²
- 2Volume = 200 × 2 ÷ 12 = 33.3 ft³
- 3Cubic yards = 33.3 ÷ 27 = 1.23 yd³
- 4Bags (1 ft³ each) = ⌈33.3⌉ = 34 bags
- 5Estimated cost = 34 × $5 = $170
4×8 ft raised bed at 4 in
A new raised bed needing a deep amendment to build the planting layer from scratch.
- 1Area = 8 × 4 = 32 ft²
- 2Volume = 32 × 4 ÷ 12 = 10.67 ft³
- 3Cubic yards = 10.67 ÷ 27 = 0.40 yd³
- 4Bags = ⌈10.67⌉ = 11 bags
30×30 ft lawn top-dressing at 0.5 in
An existing lawn given a thin top-dressing after aeration to feed grass and improve soil.
- 1Area = 30 × 30 = 900 ft²
- 2Volume = 900 × 0.5 ÷ 12 = 37.5 ft³
- 3Cubic yards = 37.5 ÷ 27 ≈ 1.39 yd³ — bulk delivery is usually cheaper than bags at this size
Introduction
The Compost Calculator tells you exactly how much compost to buy or make to cover your garden, lawn, or raised bed at the right depth. Quality compost adds organic matter, builds soil structure, holds water, and feeds beneficial soil life. Enter the area's length and width in feet plus a depth in inches, and the calculator returns the volume in cubic feet and cubic yards, the number of 1 ft³ bags needed, and a quick cost estimate. For related growing tools, see our mulch calculator, fertilizer calculator, and bulb spacing calculator.
How the Compost Volume Is Calculated
The math is unit conversion: depth is given in inches but length and width are in feet, so depth is divided by 12 to convert it to feet before multiplying. The result is cubic feet, which divides by 27 to give cubic yards (the unit used for bulk delivery).
Volume (ft³) = Length (ft) × Width (ft) × Depth (in) ÷ 12
Cubic yards = ft³ ÷ 27 (1 yd³ = 3 ft × 3 ft × 3 ft = 27 ft³)
1 cubic yard covers about 162 ft² at 2 inches deep
Bags (1 ft³ each) = ft³ rounded up to the next whole bag
Estimated cost uses $5 per 1 ft³ bag — adjust mentally for your local pricing
Always order ~10% extra to allow for settling and uneven spreading
How to Use This Calculator (Step by Step)
You only need three numbers. The calculator handles all unit conversions for you.
Measure the bed or lawn area in feet — Length × Width gives the area in square feet
For irregular shapes, break the area into rectangles, run each through the calculator, then add the results
Pick a depth in inches from the chart below (top-dressing is thin; new beds are thicker)
Read the volume in cubic feet for bagged compost, or cubic yards for a bulk delivery
Use the bag count and cost as a sanity check before placing an order
Add 10% to your final number — settling and edges always eat more than the math suggests
Buying more than ~1 cubic yard? Bulk delivery is typically 50–70% cheaper per cubic yard than bags.
Recommended Compost Depth by Application
The right depth depends on what you're growing and your soil's current condition. These ranges follow university extension recommendations.
| Application | Light (in) | Medium (in) | Heavy (in) | Notes |
|---|---|---|---|---|
| New garden / raised bed | 1 | 1.5 | 2 | Mix into top 6–8 in of native soil |
| Existing garden | 0.5 | 1 | 1.5 | Top-dress, then lightly work in |
| New lawn seeding | 0.5 | 0.75 | 1 | Mix into top 4–6 in |
| Existing lawn (top-dress) | 0.25 | 0.375 | 0.5 | Apply after aeration |
| Flower bed | 1 | 1.5 | 2 | Seasonal amendment |
| Vegetable garden | 1.5 | 2 | 3 | Heavy feeders need more |
| Trees & shrubs | 2 | 3 | 4 | Spread under root zone, keep off the trunk |
Bagged vs Bulk: Which Should You Buy?
The crossover point is roughly one cubic yard. Below that, bags are convenient; above, bulk wins on price and time.
| Format | Best for | Typical price | Pros & cons |
|---|---|---|---|
| 1 ft³ bag | < 0.5 yd³ projects | ~$5 per bag | Easy to carry; expensive per yard; lots of plastic |
| 2 ft³ bag | 0.5–1 yd³ projects | ~$8–10 per bag | Most common retail size; ~13.5 bags = 1 yd³ |
| Bulk delivery (truck) | ≥ 1 yd³ projects | ~$30–50 / yd³ | Cheapest; needs space for the dump pile and a driveway |
| U-haul / scoop at yard | 1–3 yd³ projects | ~$25–40 / yd³ | Cheap if you have a pickup; manual loading |
1 yd³ ≈ 13.5 standard 2 ft³ bags ≈ 27 standard 1 ft³ bags. A finished cubic yard weighs roughly 1,000–1,600 lb.
Choosing Quality Compost
Not all compost is equal. Good compost is dark, crumbly, smells earthy (not sour or like ammonia), and has no recognisable original materials.
Look for USCC STA certification — independently tested for maturity, contaminants, and pathogens
C:N ratio of finished compost should be ~15:1 to 25:1
pH 6.0–8.0 (near neutral) suits most plants
Avoid product that smells like ammonia (unfinished, too high in nitrogen)
Mushroom compost is alkaline — great for veg, but skip it for blueberries, azaleas, and other acid lovers
Avoid compost made from biosolids, treated wood, or unknown sources for food crops
How to Apply Compost
Once you have the right amount, technique matters. Spread evenly and avoid common mistakes.
Wait until soil is workable (not wet enough to clump) before spreading
Top-dress and rake to spread evenly — a level layer matters more than a thick one
On garden beds, lightly fork into the top 4–8 in; do not bury too deep
On lawns, top-dress no more than 0.25–0.5 in at a time; thicker layers smother grass
Around trees and shrubs, spread under the canopy but keep compost off the trunk/bark
Water after spreading to settle the layer and start microbial activity
Common Mistakes to Avoid
Most under-/over-buying traces back to a handful of repeat errors:
Forgetting to convert inches to feet — applying the formula without ÷ 12 over-orders by 12×
Buying too little because you didn't add 10% for settling and spillage
Using bags for projects > 1 yd³ — usually 2–3× the cost of bulk
Piling compost too thick on lawns (smothers grass) or against tree trunks (rots bark)
Confusing compost with mulch — for surface mulching, use a mulch calculator and pick a coarser product
Applying unfinished compost — can rob nitrogen as it continues to decompose in the bed
Compost vs Mulch vs Topsoil
These three soil products are often confused. They look similar but do different jobs.
| Product | What it is | Used for | Typical depth |
|---|---|---|---|
| Compost | Decomposed organic matter | Soil amendment, nutrient & microbe boost | 0.25–3 in worked in |
| Mulch | Coarse organic material (bark, wood chips, straw) | Surface cover to suppress weeds and hold moisture | 2–4 in on top, not mixed in |
| Topsoil | Mineral soil, often blended with organic matter | Building beds and filling holes | 4–12 in (the planting layer) |
Compost & Soil Glossary
Quick definitions for the terms used in this calculator and on compost product labels:
| Term | Definition |
|---|---|
| ft³ (cubic foot) | A box 1 ft × 1 ft × 1 ft. Compost bags are usually 1 or 2 ft³. |
| yd³ (cubic yard) | A box 3 ft × 3 ft × 3 ft = 27 ft³. The standard bulk delivery unit. |
| Top-dress | Spreading a thin layer on the surface without digging it in. |
| Amendment | Material worked into existing soil to improve it (compost, peat, perlite). |
| C:N ratio | Carbon-to-nitrogen ratio — finished compost is ~15:1 to 25:1. |
| STA certification | US Composting Council's independent compost-quality program. |
| Settling | Volume loss as a fresh compost layer compresses (typically 20–30%). |
Quick Reference Card
Compost — Quick Reference
Quick reference • Compost Calculator
ft³ = L × W × d ÷ 12 • yd³ = ft³ ÷ 27 • bags = ⌈ft³ ÷ 1⌉Valid range: Depth typically 0.25–4 in; ±10% for settling
Common Values
⚠ Watch Out
- •Don't forget to convert depth from inches to feet (÷ 12) before applying the formula
- •Don't pile compost thicker than 0.5 in on an established lawn — it will smother grass
- •Keep compost off tree trunks and stems to prevent rot
- •Don't apply more than ~4 in per year to a bed — phosphorus and salts can build up
Pro Tips
- →Order ~10% extra to cover settling (20–30%) and edges
- →Switch from bagged to bulk delivery once you're buying ≥ 1 cubic yard
- →Top-dress in spring or autumn, then water in to settle the layer
- →For irregular shapes, break the area into rectangles, calculate each, then add
FAQs
How much compost do I need per square foot?
At a standard 2-inch depth, you need 0.167 ft³ per square foot (2 ÷ 12). For 100 ft² at 2 in deep that's 16.7 ft³, or about 0.62 cubic yards. The calculator does this for you and adds bag count and an estimated cost.
How do I calculate compost in cubic yards?
Compute cubic feet first: ft³ = Length (ft) × Width (ft) × Depth (in) ÷ 12. Then divide by 27 to convert to cubic yards. For a 20 × 10 ft bed at 2 in deep, that's 200 × 2 ÷ 12 = 33.3 ft³, or 33.3 ÷ 27 = 1.23 yd³.
How deep should compost be for a vegetable garden?
About 1.5–3 inches per year for vegetables, worked into the top 6–8 inches of soil. Heavy feeders such as tomatoes, squash, and corn benefit from a 2–3 inch refresh each season; lighter feeders such as beans and herbs do well with 1–1.5 inches.
Should I buy bags or bulk compost?
Under about 1 cubic yard, bags are convenient. Above that, bulk delivery is usually 50–70% cheaper per cubic yard and much faster to handle. As a rule of thumb, 1 yd³ ≈ 13.5 standard 2 ft³ bags.
Can you add too much compost?
Yes. Applying more than ~4 inches in a year can build up phosphorus and salts and create nutrient imbalances. Most gardens thrive on 1–3 inches per year, and lawns on no more than 0.25–0.5 inches at a time.
How much extra compost should I order?
Add about 10% to your calculated volume. Fresh compost settles 20–30% in the first few weeks, edges and uneven ground always eat more than the math suggests, and being slightly over is much better than running out mid-project.
What's the difference between compost and mulch?
Compost is fully decomposed organic matter that you mix in or top-dress thinly to feed soil and plants. Mulch is coarser material (bark, wood chips, straw) you spread on the surface to suppress weeds and hold moisture. For surface mulching, use a [mulch calculator](/biology/mulch-calculator) and a coarser product.
How much does a cubic yard of compost weigh?
Roughly 1,000–1,600 pounds depending on moisture and ingredients. That's why bulk delivery makes sense: a single yard would be a few hundred 1 ft³ bags by weight, all in one drop.
Can I use this calculator for mulch or topsoil?
The volume math is the same — Length × Width × Depth ÷ 12 in cubic feet — so the bag and yard counts are correct for any loose soil amendment. Only the recommended depth changes (mulch is usually 2–4 in, topsoil for new beds 4–12 in).