Garden and Outdoor Calculators for Real Gardens
Garden planning has two classic failure modes. Too little, which means an unscheduled drive to the garden centre halfway through a job, and far too much, which means several tonnes of soil sitting under a tarpaulin on the drive for a month. Both usually come from treating the ground as a neat rectangle it almost certainly is not. The twenty two calculators here replace that guess with numbers you can take off a tape and a length of string.
None of them is a generic estimator. Each models the thing actually being measured: the spread of a hedge rather than the length of it, the taper of a tree planting hole rather than a cylinder, the way a compost heap loses nearly half its height as it breaks down. And each one leaves the assumption you might reasonably disagree with visible as an input, whether that is mulch depth, seed spacing or how long your soil takes to drink a millimetre of water.
Choose the calculator by the job in front of you
01Planning the plot
Shape first, quantity second. These split awkward beds into pieces you can actually measure, then convert that area into how many plants, seeds or rolls of turf fit at the spacing you intend to use.
02Soil, compost and feeding
Soil is sold by volume and behaves by weight. These cover depth, settling, shrinkage and the difference between what a container holds and what the roots can actually reach.
03Hard landscaping
Patios, paths, fences and greenhouses, where the expensive part is usually what sits underneath rather than what you can see.
04Watering
Flow rates and soil intake rather than sprinkler folklore. Useful both for sizing a new system and for working out how long to leave an existing one running.
05Green waste
What comes out of a hedge at the end of an afternoon, expressed in something you can actually dispose of.
What these calculators assume about your garden
Shape first, and honestly. A garden is rarely one rectangle. The area tools here split an awkward plot into pieces that can each be measured: a rectangle for the main body, a triangle for the corner the path cuts off, a circle for the bed around the tree. Adding those together takes a couple of minutes and is routinely ten or fifteen percent away from multiplying the longest two sides, which is exactly the difference between ordering right and ordering twice.
Spacing, and why halving it quadruples the order. Plants per square metre follows from spacing, and it goes up with the square of any reduction. Going from 400 mm to 300 mm is not twenty five percent more plants, it is roughly seventy seven percent more. That single relationship explains most of the sticker shock people feel when they tighten a planting plan, and it is why these calculators show both the total and the per row count.
Volume versus weight versus what arrives. Soil, compost and mulch are sold by the litre or cubic metre but handled by weight, and they change their minds about volume constantly. A compost heap commonly loses forty to sixty percent of its height as it breaks down. Excavated soil bulks above the volume it occupied in the ground. Mulch settles after the first heavy rain. Where those three behaviours matter, the calculator applies the factor rather than pretending the cubic metre is a cubic metre.
Water follows the soil, not the schedule. Irrigation is sized by both supply and uptake. Emitter flow and spacing give you litres per hour; the soil type gives you how many millimetres per hour it will accept before water runs off sideways; root depth gives you how deep the water needs to get. The run time that comes out of combining those three is usually shorter and more frequent than what people set by habit, which is why so much irrigation water ends up below the roots or down the path.
Time is a real input. Several tools here are seasonal. Frost dates set sowing windows, and a three week difference between your micro-climate and the published average changes whether a tender crop succeeds. Compost takes months, not weeks. Turf has to be laid within a day or two of delivery. The numbers include those constraints rather than pretending the job happens instantly.
A planning estimate cannot see your substrate, your weather or how square the room is. Treat every result here as a number to order against with a margin, not as a contract, and re-measure anything expensive once the old covering is off.
Reference values to start from
| Subject | What it depends on | Typical starting point |
|---|---|---|
| Mulch depth | What you are suppressing, what is planted underneath | 50 to 75 mm for weed control; under 40 mm lets light through, over 100 mm can starve the soil |
| Compost shrinkage | Feedstock mix, turning, moisture | A heap commonly loses 40 to 60 percent of its volume as it breaks down |
| Bulb spacing | Bulb size, whether you want drifts or rows | Around three bulb widths apart; halving spacing quadruples the count |
| Seed rate | Seed size, germination percentage, thinning | Small seed in grams per ten metre row; large seed spaced every few centimetres |
| Drip emitters | Emitter rating, spacing, zone pressure | Two litres per hour emitters at about 300 mm spacing is common |
| Soil bulking | Soil type, how long it has been disturbed | Excavated soil can bulk 10 to 40 percent above the volume it occupied in the ground |
A worked example: laying out a new vegetable bed
Six metres by three, dug out of lawn, in a garden with no existing irrigation. Here is everything that needs ordering before the first plant goes in.
- Area and soil volumeEighteen square metres at a working depth of 300 mm is 5.4 cubic metres. That is ninety bags at sixty litres, or one bulk delivery, and somewhere around seven tonnes to move either way. Knowing the weight up front changes the decision from how many bags fit in the car to whether you can get a lorry near the gate.
- Mulch on topSixty millimetres over eighteen square metres is just over one cubic metre: roughly fifteen large bags. Less than fifty and you will be fighting weeds all summer; more than a hundred and the surface starts shedding water instead of taking it.
- Plants at your spacingAt 300 mm square spacing the bed takes about eleven plants per square metre, which is close to two hundred for lettuce and wildly too many for brassicas. Shift to 450 mm and you are back near ninety. Same soil, same mulch, half the plants.
- Water to suit the row layoutRows at 450 mm across the short axis give four lines of six metres, twenty four metres of line in total, and about eighty emitters at 300 mm spacing. At two litres per hour that is 160 litres an hour, comfortably inside one zone for most domestic supplies.
- Then check the seasonSowing dates come off your last frost rather than off the calendar. If your last spring frost runs past the middle of spring, half the crops on the plan need to start indoors or simply start later, and the harvest window shifts with them.
Where these estimates come apart
- Measure the awkward bits separately instead of averaging the whole plot
- Order soil and mulch by volume but plan how you will move the weight
- Check your last frost date against the seed packet rather than the calendar
- Split irrigation into zones on flow rate, not on how many beds you have
- Leave a path wide enough for a barrow, including when everything is wet
- Halve a spacing recommendation without looking at the total plant count
- Assume a compost heap keeps the volume you filled it with
- Buy turf before you know when you can lay it, it is perishable
- Water to a schedule rather than to what the soil can absorb
- Fill a container to the brim and expect the roots to reach the top