Built by gardeners, for gardeners — free tools that give you real answers.
GardenCalc started from a simple frustration: trying to figure out how many tomato plants would fit in a new 4×8 raised bed, and finding only vague advice like ‘plant 18-24 inches apart.’ That answer requires math most people don’t want to do while standing in the garden with a tape measure. So we built a calculator that does it in two seconds.
Every tool on this site is designed around a single question a real gardener actually asks — not what a database administrator thinks sounds useful. Plant spacing, soil volume, frost dates, planting calendars, companion planting compatibility — these are the questions that come up in the first season of gardening and keep coming up every season after that.
We keep everything free and require no account creation because the people who benefit most from accurate gardening math are often first-time gardeners on a tight budget. Paying $12/month to find out how much mulch to buy doesn’t make sense.
We don’t make numbers up. Every calculation uses verified, publicly available data.
Frost dates and growing season lengths are based on NOAA's official 30-year climate normals dataset, covering all US weather stations. These are the same averages used by professional growers, seed companies, and university extension services.
Hardiness zone lookups use the USDA's 2023 revised Plant Hardiness Zone Map, which updated zone boundaries based on more recent temperature data. The 2023 edition shifted some areas by half a zone compared to the previous 2012 version.
Plant spacing, soil amendment rates, and fertilizer application formulas are sourced from Cornell Cooperative Extension, Purdue Extension, and UC Cooperative Extension — the most rigorously tested sources for home gardening guidance in the US.
Days-to-maturity figures for vegetables are cross-referenced against multiple commercial seed catalogs including Johnny's Selected Seeds and Baker Creek Heirloom Seeds to ensure accuracy across common varieties.
When we build a new calculator, we start with the formula from at least two independent sources — typically a university extension publication and a peer-reviewed horticultural reference. If the sources disagree, we use the more conservative estimate and note the discrepancy.
We update tools when underlying data changes. For example, the USDA released a revised Plant Hardiness Zone Map in November 2023 — we updated our zone lookup data within two weeks of that release.
All tools include a “How We Calculated This” section explaining the exact formula used. We believe gardeners should be able to verify our math, not just trust it.
Found an error in one of our calculators? Have a tool idea? Want to report inaccurate data? We genuinely want to hear from you.
Email us at hello@gardencalc.com
Send Us a Message →