Our approach
Every calculator on this site shows its formula on the page, so you can check the maths yourself with a pocket calculator. We use standard arithmetic for volumes, areas and conversions — length × width × depth for volume, area × depth × 0.623 for gallons of water, and so on. Where a result depends on a recommendation — a seeding rate, a fertilizer rate, a spacing — we tell you to take that recommendation from the seed packet, a soil test, or your local extension service, because the right number varies by crop, soil and region. A calculator cannot know your soil.
Formulas used on this site
- Soil / mulch / gravel volume: cubic feet = length (ft) × width (ft) × depth (in) ÷ 12; cubic yards = cubic feet ÷ 27.
- Plant spacing: square layout = area ÷ spacing (ft)²; triangular layout divides additionally by 0.866.
- Seed quantity: seeds = plants wanted ÷ germination fraction, rounded up.
- Fertilizer: product (lb) = (lb nitrogen per 1,000 sq ft × area ÷ 1,000) ÷ (nitrogen % ÷ 100), using the first NPK number.
- Watering: gallons = area (sq ft) × inches × 0.623; litres = gallons × 3.785.
- Container soil: cubic feet = π × radius² × height ÷ 1,728, with radius and height in inches.
- Compost: about 2–3 parts browns to 1 part greens by volume, as a practical rule for a pile near 30:1 carbon to nitrogen — not a precise chemical formula.
- Seed starting: sowing date = last frost date minus weeks before transplanting from the seed packet.
- Lawn seed: pounds = area ÷ 1,000 × rate (lb per 1,000 sq ft).
- Soil mix: each component = total volume × its percentage ÷ 100.
- Pavers: count = area ÷ (paver size in feet)², rounded up, plus 10% waste.
- Drip irrigation: gallons per week = emitters × flow rate (gal/hr) × hours per week.
- Harvest yield, soil cost, row planting: simple multiplication of the inputs you enter, shown on each page.
What we check before publishing
Each calculator is tested with a worked example that we calculate by hand and compare with the tool's output. Each guide is written from general gardening practice, not copied from another site, and is checked for statements that would only be true in one climate or for one crop. Pages are reviewed when we add a tool or when a reader points out an error — corrections are welcome via the Contact page, and we fix verified errors rather than defending them.
What calculators cannot do
A calculator cannot test your soil, feel its moisture, or know your local frost dates. It cannot replace a soil test for fertilizer decisions, a rain gauge for watering decisions, or the seed packet for spacing and sowing depth. Treat every result as an estimate to check against what you see in your own garden, and follow local extension guidance where it differs from a general rule on this site. See the Disclaimer.
Units
We work in imperial units first (feet, inches, gallons, pounds) because that is how most US seed packets, fertilizer labels and supplier quotes are written, and we show metric conversions where they help. Enter units exactly as each calculator asks — mixing feet and inches is the most common error, and each page states its units in the field labels.
Last updated: 4 November 2026.