Eleven calculators for people who grow things

Frost datesAlmost every date in a growing year is counted from the last spring frost. This gives you th… Planting calendarPick a crop and a zone and this works out when to sow under cover, when to plant out, when t… Raised bedsLength times width times depth, split into a mix that will still behave like soil in three y… CompostArea times depth, converted into bags and bulk bags, with the weight as well — because compo… SpacingChoose a crop and the recommended spacings fill themselves in; change the bed size and layou… WateringHow much water a bed actually needs this week, after subtracting the rain that has already f… Seed startingWhen to sow under cover, when to plant out, and how many seeds that means sowing once germin… GreenhouseGlazed area times the U-value of the glazing times the temperature you are lifting — which g… Soil pHHow much lime raises your pH, or how much sulphur lowers it, adjusted for soil texture — bec… LawnGrass seed by area at the right rate for the job, and the fertiliser arithmetic that turns a… RainwaterRoof plan area times annual rainfall times a runoff coefficient — which gives what you can a…

Start with the frost date

Almost every date in a growing year is counted from the last spring frost, which is why the frost date calculator sits first and why the planting calendar reads from it. Set your zone once and the sowing, transplanting and harvest dates for twenty-four crops follow from it.

The important thing to understand about that date is that it is a median rather than a deadline. Frost after the published last-frost date happens roughly one year in two. The tools here quote a safe date two weeks later as well, and that is the one to plan tender crops around.

The second thing the calendar does that most do not is check whether a crop actually fits. A tender crop has to complete its whole run between the last spring frost and the first autumn one, and in a short season plenty of them simply do not. Being told that in March is far more useful than discovering it in October.

The quantity tools

The rest are arithmetic that is easy to get wrong in expensive ways. How much soil a raised bed swallows — and why filling it with pure compost means topping it up forever. How deep a mulch has to be before it suppresses anything. How much water a bed actually needs, which is almost always several times what a watering can delivers.

Each one carries the reasoning as well as the number, because the number on its own is rarely the useful part. The rainwater calculator will tell you what your roof yields; it will also tell you to measure the footprint rather than the slope, which is the mistake that inflates most estimates by a third.

How the numbers are worked out

Frost dates are median 0 °C dates by USDA hardiness zone, with a separate maritime setting because the zone system misdescribes mild-winter, cool-summer climates like the UK, Ireland and the Pacific Northwest. Crop timings are expressed as weeks either side of the last frost, so changing your zone moves every date together.

Watering uses the crop coefficient method and the fact that one millimetre of water over one square metre is one litre. Greenhouse heat loss is glazed area times the U-value of the glazing times the temperature lift. Liming rates are adjusted for soil texture, because clay resists a pH change nearly three times harder than sand and a single published figure is close to meaningless without that.

Every tool works in both imperial and metric, and the conversion is done in one direction only — a rate quoted in one system is never applied to a measurement taken in the other.