Enter your garden bed size or compost bin dimensions. Get cubic feet, cubic yards, how many bags to buy, and whether your carbon-to-nitrogen ratio is in range — all at once.
Compost Calculator
🌿 Ideal C:N ratio: 30:1 (carbon to nitrogen). Browns (dry leaves, straw, paper) provide carbon; Greens (food scraps, grass, manure) provide nitrogen. Mix by weight: ~2 parts browns to 1 part greens.
💡 Pro tip: For best results, chop materials small, keep pile moist (like a wrung sponge), and turn weekly. Avoid meat, dairy, and oils.
What this calculator figures out
Most compost calculators answer one question and stop. This one handles three things people actually get wrong:
Volume and bag count. Enter your bed’s length, width, and compost depth. The calculator returns cubic feet, cubic yards, and the exact number of 1 cu ft, 1.5 cu ft, and 2 cu ft bags to buy — rounded up so you don’t run short at the nursery.
Carbon-to-nitrogen ratio. If you’re building a pile from scratch, enter your browns (dry leaves, cardboard, straw) and greens (kitchen scraps, grass clippings, coffee grounds) by weight. A healthy pile targets 25:1 to 30:1. The calculator tells you whether your mix is there, too nitrogen-heavy, or too carbon-heavy, and what to add to fix it.
Compost pile volume from bin size. Got a 3×3×3 ft bin? Enter those dimensions and a fill percentage, and you’ll see total pile volume alongside your greens-to-browns breakdown
How to use the compost calculator
Mode 1 — How much compost do I need? (volume calculator)
- Measure your bed or area: length and width in feet, or inches if that’s easier.
- Choose your compost depth (see the table below if you’re unsure).
- Hit Calculate. The result shows cubic feet, cubic yards, and bag counts by size.
- Add 10–15% to your order to account for settling.
Mode 2 — Is my compost pile balanced? (C:N ratio calculator)
- Weigh or estimate your greens (nitrogen-rich materials).
- Weigh or estimate your browns (carbon-rich materials).
- Enter both weights. The calculator shows your current C:N ratio and how far it is from the target 30:1.
- If the ratio is off, the tool tells you roughly how much to add to correct it.
You don’t need a scale to get a useful estimate. A standard kitchen compost bin holds about 3–5 lbs of scraps. A 30-gallon trash bag of dry leaves weighs roughly 10–15 lbs. Use these as rough anchors if you haven’t weighed anything.
How deep to apply compost (by project type)
The right compost depth depends on what you’re trying to do. These are the same ranges used by university extension programs — not rules carved in stone, but a solid starting point.
| Use case | Recommended depth | Notes |
|---|---|---|
| Vegetable garden (new bed) | 2–4 inches | Work into top 6–8 inches of existing soil |
| Vegetable garden (established) | 1–2 inches | Top-dress in spring or after harvest |
| Flower bed (established) | 1–2 inches | Spread around plants, avoid crown contact |
| Raised bed (new, filling) | 25–30% of total volume | Blend with topsoil and other amendments |
| Raised bed (top-up) | 1–2 inches | Each season as beds compact and settle |
| Lawn top-dressing | ¼ to ½ inch | Best done after aeration in fall or spring |
| Tree and shrub planting hole | Up to 25% of backfill | Don’t exceed 25% — roots need native soil contact |
| Potting mix (containers) | Up to 25% of mix volume | Pure compost in pots holds too much moisture |
For a standard 4×8 raised bed filled to 12 inches, you’d need roughly 32 cubic feet of material total — about 8–10 cubic feet of compost if you’re targeting 25–30% of the mix.
C:N ratio guide
Carbon-to-nitrogen ratio: why it matters and how to fix it
The C:N ratio describes the proportion of carbon (browns) to nitrogen (greens) in your pile. Microbes need both to break down organic matter efficiently. Too much nitrogen and the pile heats fast but turns sour or releases ammonia. Too much carbon and the pile barely heats at all — it sits there doing nothing for weeks.
The target is roughly 30 parts carbon to 1 part nitrogen (30:1). You don’t need to hit it exactly — most piles somewhere between 25:1 and 35:1 will decompose at a reasonable pace.
Common materials and their approximate C:N ratios
| Material | Type | Approx. C:N ratio |
|---|---|---|
| Kitchen scraps (mixed) | Green | 15:1 |
| Fresh grass clippings | Green | 15–25:1 |
| Coffee grounds | Green | 20:1 |
| Fresh manure (cow) | Green | 15–20:1 |
| Dry leaves (deciduous) | Brown | 40–80:1 |
| Straw | Brown | 40–100:1 |
| Cardboard (shredded) | Brown | 350–500:1 |
| Sawdust (untreated) | Brown | 200–500:1 |
| Wood chips | Brown | 100–400:1 |
A practical starting point: two parts browns to one part greens by weight. This gets most home piles close to 30:1 without detailed math. The calculator lets you go further if you want a more precise number.
What to do if your ratio is off
If the calculator shows a ratio below 20:1 (too nitrogen-heavy): add dry leaves, shredded cardboard, or straw. Roughly 20–25 lbs of dry leaves per 10 lbs of excess greens usually brings a pile back into range.
If the ratio is above 40:1 (too carbon-heavy): add fresh grass clippings, kitchen scraps, or a small amount of blood meal or feather meal. The pile will also benefit from extra water — carbon-heavy piles tend to dry out faster.
Bags vs. bulk: which makes sense for your project
The break-even point for most gardeners is around 3–4 cubic yards. Below that, bags from a garden center are convenient and usually cost-effective. Above that, a bulk delivery of loose compost almost always costs less per cubic yard — sometimes by 40–60%.
| Bagged compost | Bulk cubic yard delivery | |
|---|---|---|
| Best for | Beds under 100 sq ft, top-ups, containers | New builds, multiple beds, full fills |
| Typical cost | $6–$12 per 1.5 cu ft bag | $25–$65 per cubic yard (varies by region) |
| Equivalent bags per yard | ~18 bags of 1.5 cu ft | — |
| Main downside | Expensive at scale, lots of plastic | Requires space to pile, may need wheelbarrow |
One cubic yard covers about 162 square feet at a 2-inch depth, or 324 square feet at 1 inch. For a 4×8 raised bed filled 3 inches deep, you need roughly 8 cubic feet — that’s 6 bags of 1.5 cu ft, or just under half a cubic yard in bulk.
Always order 10–15% more than the calculator recommends. Compost compresses as it settles, especially in the first month after watering. One short bag means a second trip.
Compost for raised garden beds: what to use and how much
Raised beds are where compost makes the biggest difference. Native soil compacts inside a frame; compost keeps the mix loose and biologically active over multiple seasons.
For a new raised bed filling from scratch, target 25–30% compost in the overall mix. A typical recipe: 60% topsoil (or a structured mix), 30% compost, 10% perlite or coarse sand for drainage. If you’re using Mel’s Mix or a similar soilless approach, the ratios shift — but the calculator still handles the volume math once you know the compost percentage you’re targeting.
For beds that have been growing for a year or two: the soil has compressed and the organic matter has been used up. Add 1–2 inches of fresh compost on top each spring before planting, and work it into the top few inches if the surface has crusted. This is also when to check whether the bed needs a pH correction — compost itself is usually close to neutral (6.5–7.0), which works for most vegetables.
Compost should make up no more than 30% of a container mix. In pure compost, roots struggle with moisture retention and lack of structure. A pot filled entirely with compost tends to stay wet at the bottom and dry out fast at the surface.
Compost for lawns: top-dressing and overseeding
Lawn compost applications are much thinner than garden bed applications — ¼ to ½ inch is standard. At that depth, one cubic yard covers between 650 and 1,300 square feet, which makes bulk delivery worth considering even for a mid-sized lawn.
Timing matters: apply in early spring or fall when grass is actively growing. For overseeding, spread compost first, then broadcast seed over it, or reverse the order — either works. The compost keeps the seed moist during germination and gives new grass a nutritional boost without burning it.
Core aeration before top-dressing gets compost into the root zone faster. If you aerating, the plugs break down on their own — no need to rake them up before spreading compost.
FAQ
Frequently asked questions
How much compost do I need for a 4×8 raised bed?
For a standard 4×8 ft bed with 12 inches of total depth, targeting 25% compost in the mix, you need about 8 cubic feet — roughly 6 bags of 1.5 cu ft. For a 3-inch compost top-dressing on an existing bed, you need about 8 cubic feet as well (4 ft × 8 ft × 0.25 ft depth). The calculator handles both scenarios.
What is the ideal C:N ratio for compost?
Most composting references target 25:1 to 30:1. At that range, microbial activity is high, the pile heats well, and decomposition takes 2–6 months depending on pile size, moisture, and turning frequency. Below 20:1 the pile tends to smell; above 40:1 it stays cool and takes much longer to break down.
How many cubic yards is a cubic foot?
One cubic yard equals 27 cubic feet. So if the calculator returns 27 cubic feet, that’s exactly one yard — the most common bulk ordering unit. Divide any cubic feet result by 27 to get cubic yards.
How much does a cubic yard of compost weigh?
Compost weight varies significantly by moisture content. Dry compost runs around 700–900 lbs per cubic yard. Wet or freshly processed compost can weigh 1,200–1,600 lbs per yard. Most suppliers quote weight ranges on their product pages, and it matters mainly for delivery — some trucks limit loads to 1–2 yards per trip on soft driveways.
Can I use compost instead of topsoil?
Not as a direct replacement in beds or lawns. Pure compost holds too much water, compresses as it breaks down, and lacks the mineral structure that roots need for anchoring. It works best blended at 25–30% with topsoil or another structural medium. The one exception is a fully soilless mix (like Mel’s Mix), which uses equal parts compost, peat moss or coco coir, and coarse vermiculite — though this is expensive at scale.
How much compost to add to garden soil that’s already established?
One to two inches worked into the top 6 inches of soil, once or twice a year, is enough for most vegetable gardens. Sandy soils benefit from the higher end; heavy clay soils see bigger improvement but also take longer to incorporate amendments. You don’t need to add compost every season if the soil is already in good shape — check texture and drainage first.
What should I not compost?
Meat, fish, dairy, and cooked foods with oils attract pests and create odor problems in most home setups. Diseased plants can spread pathogens if the pile doesn’t reach high enough temperatures. Dog and cat waste contain parasites and pathogens not killed in typical home compost conditions. Treated wood products (pressure-treated lumber sawdust, painted wood) contain chemicals you don’t want in garden soil.
How long does compost take to be ready?
A hot, actively managed pile (turned weekly, kept moist, good C:N ratio) can produce finished compost in 2–3 months. A cold pile that you add to occasionally and leave alone takes 6–18 months. Finished compost looks dark and crumbly, smells earthy, and you can’t identify the original materials anymore.