Scottish growing tool
Bed Area Calculator
Measure your bed, and this works out its area and how much garden compost or well-rotted manure to add: dug in or as a surface mulch.
Sources
Digging-in and mulching rates, and the compost/Growmore nutrient comparison:
Mulch depth and bag-size corroboration:
Why there's no numeric fertiliser rate here: general granular feeds are sold with a nitrogen/phosphorus/potassium strength printed on the bag, but no standard application rate, since that depends on the specific product, so this tool doesn't invent a figure no one else publishes.
Cost estimate: bagged manure/soil-improver prices checked live at two major UK retailers on 2026-08-05. Prices move with stock and offers, so treat this as a ballpark, not a quote:
How it works
The calculator works out the bed's area, length times width for a rectangle, or from the diameter for a round bed, then applies the RHS's rates for organic matter. Dug in, that's 5 to 10kg of garden compost or well-rotted manure per square metre, which is half to one 15-litre bucket. Spread on top as a mulch, it's 50 litres per square metre for a 5cm layer, the minimum that keeps weeds down. The RHS suggests 8cm (3in) is easier and more effective, at about half as much again: 75 litres. Mulch is counted in 70-litre bags, rounded up, since you can't buy part of a bag.
A worked example
A standard raised bed of 3m by 1.2m is 3.6 square metres. To dig organic matter in, it needs 18 to 36kg, about two to four buckets. For a 5cm mulch it needs 180 litres: three 70-litre bags, weighing about 120kg wet and costing roughly £25 to £47 at the bagged prices checked at two UK retailers in August 2026. An 8cm mulch takes 270 litres, four bags.
What it won't tell you
Compost and manure improve the soil's structure and feed its life, but they're not a fertiliser. Well-rotted manure is about 0.6% nitrogen and garden compost about 0.5%, against roughly 7% in a general feed like Growmore: about 1300g of manure or compost does the work of 100g of Growmore. There's no standard dose for general fertilisers, which vary by product, so the calculator doesn't invent one. If you're buying rather than making your own, look for a peat-free label: Scotland's peatlands are a major carbon store.