Compost Calculator
Choose between two modes: the Application mode calculates the volume, weight, bag count, and cost of compost you need for any garden bed, lawn, or raised bed in metric or imperial units. The Recipe mode evaluates your compost pile ingredients, calculating the combined carbon-to-nitrogen ratio and moisture content so you can adjust your mix before you build the pile.
How to calculate how much compost you need
The basic formula is straightforward: Volume = Area x Depth. Measure the length and width of your bed or lawn in feet, multiply to get the area in square feet, then multiply by the desired depth in feet (divide inches by 12 to convert). For example, a 20 ft by 10 ft bed covered with 2 inches needs 20 x 10 x (2/12) = 33.3 cubic feet of compost. One cubic foot weighs roughly 40 lb for mature bagged compost, or 650 kg per cubic metre for bulk material. If you are ordering bulk compost, convert to cubic yards by dividing cubic feet by 27 - so 33.3 cu ft is about 1.2 cubic yards. The application mode of this calculator handles all of those conversions automatically.
How to use the C:N recipe calculator
Switch to Recipe mode and choose two or three materials from the dropdown menus. Enter the relative volume of each material in any consistent unit, such as buckets, wheelbarrows, or cubic feet. The calculator works out the volume-weighted carbon-to-nitrogen ratio and moisture content of your blend. A C:N ratio between 25:1 and 40:1 is the target for active hot composting: below that range the pile becomes wet and smelly (too much nitrogen), above it decomposition stalls (too little nitrogen). Moisture between 50 and 60 percent is ideal - the pile should feel like a wrung-out sponge. If either value is out of range, adjust your proportions and recalculate until both fall in the optimal zone.
Browns, greens, and the C:N ratio explained
"Browns" are carbon-rich materials: autumn leaves (C:N roughly 60:1), straw (80:1), cardboard (350:1), and wood chips (400:1). "Greens" are nitrogen-rich: grass clippings (20:1), food scraps (17:1), and fresh manure (7-30:1 depending on type). A pile made entirely of leaves will have a C:N around 60:1 and will barely heat up. A pile of pure grass clippings at 20:1 will turn slimy. The classic advice of 3 parts browns to 1 part greens by volume is a rough starting point, but actual material characteristics vary widely, which is why a calculator that lets you select specific materials gives a much more accurate result.
Compost quality, bulk density, and buying tips
Finished mature compost is dark, earthy-smelling, and crumbly. Its bulk density typically ranges from about 400 kg/m3 for fluffy home-made compost to 800 kg/m3 for fine, dense municipal compost. Bagged products at garden centres are usually in the 500-650 kg/m3 range. This calculator uses 40 lb per cubic foot (roughly 641 kg/m3) as the default. When ordering bulk compost from a landscape supplier, ask for the C:N ratio and pathogen test results if you are using it on a vegetable garden. Bulk is almost always cheaper than bagged once you need more than about 5 cubic yards.
Compost application depth guide
| Application type | Depth (inches) | Notes |
|---|---|---|
| New lawn (seed or sod) | 2-4 | Till into top 6-8 in of soil before seeding |
| Established lawn (topdress) | 0.25-0.5 | Apply after aerating; rake in lightly |
| Garden bed (annuals / veg) | 2-3 | Incorporate before planting each season |
| New garden bed (first time) | 3-6 | Mix with existing soil; rest 2-4 weeks before planting |
| Trees and shrubs (mulch ring) | 1-2 | Keep 6 in away from stem or trunk |
| Raised bed fill | 25-50% | Mix 25-50% compost with topsoil or potting mix |
Recommended compost layer depths by application type. Source: USDA Natural Resources Conservation Service.
Frequently asked questions
How much compost do I need for a raised bed?
A raised bed is typically filled with a mix of about 25-50% compost and 50-75% topsoil or potting mix. For a 4 x 8 ft raised bed that is 12 inches deep, you need 32 cubic feet total. At 25% compost, that is 8 cubic feet, or about 8 standard bags. Use the Application mode of this calculator, enter your bed dimensions, and choose the depth you want to fill. For a raised bed from scratch choose the deepest depth option or use custom depth.
What is the ideal C:N ratio for compost?
The ideal carbon-to-nitrogen (C:N) ratio for active hot composting is between 25:1 and 40:1. Below 25:1 the pile has excess nitrogen, which escapes as ammonia causing odour and nutrient loss. Above 40:1 there is not enough nitrogen for microbes to break down the carbon, so decomposition becomes very slow. Most finished compost has a C:N around 10:1 to 20:1, which is safe to apply to plants.
How deep should I apply compost to my lawn?
For an established lawn, apply 0.25 to 0.5 inches (about 6-12 mm) as a topdressing, ideally right after core aeration so the compost falls into the holes. For a new lawn being seeded or sodded, till 2-4 inches of compost into the top 6-8 inches of the existing soil before laying seed or sod.
How many cubic yards of compost do I need to order in bulk?
Most landscape suppliers sell compost by the cubic yard. Divide your required cubic feet by 27 to convert: for example, 54 cubic feet is 2 cubic yards. One cubic yard weighs about 1,200-1,800 lb (roughly half a to one tonne). Minimum delivery quantities are usually 1-2 cubic yards, and delivery fees can make small orders expensive, so consolidate multiple projects into one order.
How do I know if my compost pile moisture is right?
The classic test is the "squeeze test": grab a handful of compost material and squeeze it. It should feel like a wrung-out sponge - moist enough to hold together but not dripping water. If it crumbles to dust, add water or wet greens. If water runs out when you squeeze, mix in dry browns such as straw or cardboard. The Recipe mode of this calculator estimates the moisture content of your blend before you build the pile.
Can I add meat or dairy to my compost pile?
In a traditional backyard heap, meat, fish, and dairy products attract pests and can cause odour, so most guides advise against them. In a hot-composting system that consistently reaches 55 to 70 degrees Celsius, or in a covered tumbler or bokashi system, these materials can be composted safely. The C:N ratios for these items are similar to other high-nitrogen food scraps in the 10-15:1 range.