Calculate how much compost or soil amendment to buy for a garden bed, as a blend share or a topdress layer, in cubic yards, cubic feet or bags.
Soil Amendment Formula
Choose the task and enter a rectangle, circle diameter, or known area. Keep the depth basis explicit: adding material to existing soil differs from specifying a final combined mixture. Volume additivity is an approximation; actual mixtures may settle.
Blend with existing soil: let S = area × existing soil depth and p = amendment share of the final volume, expressed as a fraction. Amendment volume V = S × p / (1 − p). This follows from p = V / (S + V). A 100% amendment fraction cannot contain a positive existing-soil volume.
Blend with a final-volume target: V = area × final mixture depth × p. The remaining final volume is existing soil. This basis can describe a 100% imported-material volume.
Topdress: V = area × loose layer thickness. If the layer is later incorporated, its final fraction depends on the existing soil volume; dividing its thickness by the existing soil depth alone does not give the final share.
Purchase allowance: purchase volume = V × (1 + allowance percent / 100). Blank means zero. The allowance is extra material to purchase, not an instruction to apply more or a universal settling prediction.
Supplied treatment rate: product mass = treated area × recommended product mass / reference area. Enter a recommendation already adjusted to the actual product, soil, crop and depth. This mode does not generate lime or sulfur recommendations from pH or texture, substitute products, or set a repeat schedule.
Immediate organic-matter mixture: let M = existing soil dry mass, c = current soil OM fraction, t = target OM fraction, and a = compost OM fraction of dry weight. Adding dry compost mass X gives (M × c + X × a) / (M + X) = t. Therefore:
X = M × (t − c) / (a − t) Delivered mass = X / dry fraction
Soil dry mass M = area × sampled depth × soil dry bulk density. Percentages are divided by 100 to obtain fractions. The target must exceed current OM and remain below compost OM for a finite positive addition; an unchanged target needs zero addition. Delivered dry matter is dry mass divided by wet delivered mass. Optional delivered volume = delivered mass / delivered bulk density. This is uniform immediate mixing without decomposition or material loss, not a forecast of retained OM, a future soil test or years to a target.
Use Measured Soil and Product Data
Compost, manure, castings, topsoil, peat, coir, bark, sand, perlite and vermiculite differ between products. A material name does not establish a reliable bulk density, organic-matter analysis or recommended blend fraction. Enter supplier measurements for the delivered moisture condition. Compost analyses distinguish dry-matter and dry-weight OM measurements. Soil dry bulk density uses dry mass and the total sampled soil volume.
Optional bag or package size and price give purchase quantities and costs in one consistent currency. Whole packages are rounded up; applying the entire purchase can exceed the calculated quantity. Bulk costs exclude delivery and tax. Unit changes convert the current quantity rather than relabelling it.
Lime and Sulfur Require a Specific Recommendation
Lime guidance considers active and reserve acidity, crop needs and regional conditions; current pH and a texture name do not establish a universal dose. Product-quality systems such as ENP, CCE or ECCE differ. Have the recommendation adjusted to the actual product before entry, and do not adjust a product-specific rate twice.
Carbonate-rich soils can resist practical acidification. Obtain local soil-test guidance before using sulfur or another acidifying product. Follow the actual product label and local recommendation for application limits, incorporation and retesting. The calculator supplies no generic safe single dose, automatic material factors or fixed eight-week repeat schedule.
Organic-matter targets do not override nutrient, salt or practical management limits. Review compost analysis and soil tests; excess compost can add unwanted nutrients and salts. The theoretical mass balance is an accounting model, not an application recommendation.
Example Problems
Example 1: adding compost to existing soil. A 20 ft × 4 ft bed has 8 inches of existing soil. For a final 25% amendment fraction, existing volume S = 80 × 8 / 12 = 53.3333 ft³. Net amendment = 53.3333 × 0.25 / 0.75 = 17.7778 ft³ = 0.6584 yd³. An explicitly entered 10% purchase allowance gives 19.5556 ft³ = 0.7243 yd³; at 1.5 ft³ per bag, purchase 14 bags. The net loose amendment layer is about 2.6667 inches. This is an arithmetic example, not a recommended compost rate.
If 8 inches instead describes the final combined mixture, amendment = 53.3333 × 0.25 = 13.3333 ft³. With a 10% allowance this is 14.6667 ft³, requiring 10 bags of 1.5 ft³. Choosing the correct depth basis changes the answer.
Example 2: scaling a supplied product rate. Suppose a qualified recommendation for the actual product is 10 lb per 1,000 ft². For 500 ft², quantity = 10 × 500 / 1,000 = 5 lb. If sold in 40 lb packages, purchase one package but do not automatically apply all 40 lb. The assumed rate demonstrates arithmetic only; it is not a generic lime or sulfur recommendation.
Example 3: immediate OM mass balance. A sampled layer contains 1,000 kg dry soil at 2% OM. For a hypothetical 5% target with compost at 50% OM of dry weight, X = 1,000 × (0.05 − 0.02) / (0.50 − 0.05) = 66.6667 kg dry compost. At 50% dry matter, delivered wet mass = 133.3333 kg. Check: (20 kg original OM + 33.3333 kg compost OM) / (1,000 + 66.6667 kg dry mixture) = 5%. This predicts no future retention and establishes no acceptable application rate.
FAQ
How much compost should a bed receive?
Use soil tests, compost analysis, crop needs and local guidance to choose an appropriate rate. The calculator determines quantities for your chosen inputs; it does not provide a universal blend percentage, annual layer depth or safe maximum. Do not infer suitability from a material name or target OM alone.
Why does a volume layer not equal a soil-test OM increase?
Layer depth describes volume, while soil-test OM describes a fraction of dry mass. Soil dry bulk density, compost dry matter and compost dry-weight OM are separate inputs. Added compost also changes the total dry-mass denominator. Decomposition and soil changes prevent this immediate blend model from predicting a one-year OM increase or a fixed number of seasons.
Should sand be added to clay soil?
Seek local soil-management advice for the specific soil and intended use; small sand additions may not improve structure. This quantity tool does not assess drainage, texture improvement, nutrients or amendment suitability, and supplies no automatic sand or compost treatment rate.
