Raised Bed Soil Calculator

Raised bed soil volume is calculated as length times width times depth. A depth of 200 to 300 mm suits most vegetables and flowers, while deeper beds of 400 to 600 mm are needed for root crops and fruit. The RHS recommends filling raised beds with a mix of topsoil (60 to 70%), compost (20 to 30%) and sharp sand or perlite (5 to 10%) for good drainage. Enter the bed dimensions and the calculator shows soil volume in litres and bulk bags, plus a bill of materials for the frame. A 1.2 m by 2.4 m bed at 300 mm deep is a popular size, wide enough for a double row of plants but narrow enough to reach the centre from either side without stepping on the soil.

The frame estimate covers the timber or sleeper quantity needed to build the bed. Standard options include treated softwood boards (150 by 22 mm, stacked 2 to 3 high), railway sleepers (200 by 100 mm) and composite boards. Corner posts, screws and weed membrane are included in the materials list. In the worked example, a 2.4 m by 1.2 m bed built three courses high takes 6 boards at 2.4 m and 6 boards at 1.2 m, together with 4 corner posts of 350 mm cut from 50 by 50 mm timber.

Some standard sizes are worth memorising. A 1.2 m by 2.4 m bed filled to 300 mm holds 0.86 m³ of soil, a 1.2 m by 1.2 m bed at 400 mm holds 0.58 m³, and a 0.6 m by 2 m bed at 300 mm holds 0.36 m³. A UK bulk bag is typically 0.7 to 0.9 m³, so one bag covers one or two standard beds. The soil does not stay put, either. Beds sink and decompose by 25 to 50 mm a year, which means topping up with 50 to 100 litres per bed annually, or one to two retail bags.

Depth should follow what you plan to grow. Salad leaves and herbs manage in 150 to 200 mm as a minimum. Brassicas such as broccoli, cabbage and kale want about 300 mm, potatoes and a single tomato plant per bed 300 to 400 mm, and deep-rooted carrots, parsnips and beetroot 300 to 450 mm. Fruit bushes such as blueberries and raspberries need 400 to 500 mm. Height serves people as well as plants. A children's bed of 400 to 600 mm removes the bending, and the optimum no-bend height for adult comfort is 750 to 900 mm.

Materials set most of the budget. Pressure-treated pine or spruce softwood costs £40 to £120 for a 1.2 by 2.4 m bed and lasts 8 to 15 years. Cedar or larch runs to £100 to £300 per bed, premium hardwoods such as oak and sweet chestnut £200 to £500, composite or recycled plastic £150 to £400, and galvanised metal, for a modern look, £80 to £250. Brick is the most expensive option but permanent. At the cheap end, DIY scaffolding boards come in at £30 to £70 per bed, or nothing at all if they are reclaimed.

Filling can be done far more cheaply than buying topsoil for the whole volume. The Hugelkultur method puts wood and branches in the bottom 30 to 50% of the bed, where they rot down over years, with topsoil and compost in the upper 50 to 70%, saving 40 to 60% on filling cost. The lasagne method alternates layers of cardboard, manure, compost and grass clippings and is free if you do it yourself. Cheapest of all is subsoil from an excavation, free if you are already landscaping, with the upper 200 to 300 mm made up of quality topsoil and compost. A 0.86 m³ bed filled with 0.5 m³ of wood or subsoil and 0.36 m³ of topsoil comes to £18 to £40, against roughly £60 to £100 if the whole 864 litres comes from bulk bags.

Once built, the choice of crops decides how much you get back. Courgettes are the high-yield stars, with 1 to 2 plants per bed producing 8 to 15 kg, while tomatoes give 5 to 10 kg per plant and climbing French beans 3 to 5 kg per plant. Strawberries are perennial and easy. For quick wins, salad leaves give multiple cuts a year, radishes go from seed to harvest in 4 weeks, and spring onions, parsley, basil and coriander are undemanding. Sweetcorn yields little for the space it takes, pumpkins need a huge area and cauliflower is fussy, so all three are better left to a larger plot.