About SoilSums

SoilSums is a small, independent site that answers one kind of question: how much of something a garden needs. Cubic feet of soil for a raised bed. Bags of mulch at two inches. Pounds of a 10-10-10 fertilizer across four hundred square feet. How many lettuces fit in a bed four feet by eight. It is run by Muzamil Ali.

Why it exists

Most of these sums are simple arithmetic, but they are easy to get wrong in the one place it matters — usually a unit conversion, or a depth in inches multiplied by an area in square feet without dividing by twelve. Getting it wrong means either three trips to the garden centre or six unopened bags in the garage.

The existing answers online tend to fall into two camps: a calculator with no explanation of what it just did, or a long article that never gets to a number. SoilSums tries to do both on one page — the answer at the top, the working underneath.

How the calculators are built

Every tool on this site follows the same process.

1. The arithmetic lives in a plain, separate function with typed inputs and outputs — no calculation happens inside a button or a form.
2. That function gets unit tests before any interface is built, including the awkward cases: zero, negative numbers, absurdly large numbers, and the same input expressed in both imperial and metric.
3. The page then shows the formula it used, with a worked example using real numbers, so you can check the result by hand if something looks off.
4. Invalid input produces a plain-English message next to the field. You will never see NaN or an infinity symbol where a number should be.

Where the agronomic numbers come from

Some tools need more than arithmetic. Grass seeding rates, compost carbon-to-nitrogen values, lime requirements by soil texture, plant spacing, days to maturity and yield ranges are all published figures, and they vary between sources and regions.

Every one of those values is stored in a data file alongside the source it came from and a flag recording whether it has been checked against a primary reference — typically a university extension service in the United States or Canada, or the Royal Horticultural Society in the United Kingdom. Values that have not yet been verified are marked as such in the repository, and a maintenance script lists them so none get quietly forgotten.

Two things you will never find here: an invented statistic, and a personal anecdote used as evidence. If a figure has a source, the source is named. If it is a rule of thumb, it is called a rule of thumb.

What the results are and are not

Everything on this site is an estimate. Soil settles and compacts, bagged products vary in how much they actually contain, and the amount of lime a soil needs depends on buffering capacity that no online form can see. Where a decision has real consequences — amending pH in particular — a soil test from your local extension service is worth more than any calculator, including this one. The disclaimer says this in full.

How the site is paid for

SoilSums is free and needs no account. It is funded by advertising, which is kept out of the calculators themselves — you will not find an ad between an input field and its result. What data advertising involves is set out in the privacy policy and the cookie policy.

Corrections

If a number here is wrong, or a formula does not match what your extension service publishes, please say so — corrections are the most useful mail this site gets. Write to hello@soilsums.com, or use the contact page.