Quick sums for your project, the same ones our team uses on site.
How much concrete you need for a slab, footing or post, in m³ and bags.
Shape
How this is worked out
Estimate only — check quantities before ordering.
Volume is length × width × depth (or a round post), then wastage is added.
Ready-mix is rounded up to the next 0.25 m³ — check your plant’s minimum load.
Bagged mix: about 2200 kg of dry mix for every 1 m³ poured (check the bag).
Enter sizes in metres: 100 mm = 0.1 m.
How much MOT, sand, gravel, ballast or topsoil you need, in tonnes and bulk bags.
How this is worked out
Estimate only — check quantities before ordering.
Volume is length × width × depth, then the wastage is added.
Weighs about 2.1 tonnes per m³ for this material. Wet or dry piles can differ — your merchant can confirm.
MOT Type 1 figure is for compacted stone; leave a little extra for packing down.
Each bulk bag is taken as about 0.85 tonnes; bag sizes vary.
What size radiator a room needs, in BTU and watts.
Quick start
Units
Odd-shaped room? Work out each rectangle and add them up.
Outside walls (walls to the outside air)
Windows
Room above
Room below
More options
Main window faces
Exposure
Heating system
How this is worked out
Room volume (m³) × heat rate for the room × the factors below × 15% safety margin = watts. BTU/hr = watts × 3.412.
This is the usual UK room-volume method for sizing radiators. Heat rates: bathroom 45, living / dining 40, bedroom / office / hall 35, kitchen 30 W per m³.
Windows: single +40%, double no change, triple −15%. Property: new-build −30%, post-1920s cavity no change, older poorly insulated +20%, Victorian / pre-1920s +50%.
Outside walls: 1 −15%, 2 no change, 3 +15%, 4 +30% (no published figure for none — pick 1, which reads a little high). North-facing +15%, south −10%. Sheltered −10%, exposed +15%.
Unheated room or loft above, or unheated space below: +15% (counted once).
Radiator range = the need, up to 10% more (pick a radiator within about 10%, and go bigger if unsure), rounded up to 50 W at ΔT50 (the figure in radiator catalogues). It is the total for every radiator or towel rail in the room. For ΔT40 / ΔT30 it is divided by 0.75 / 0.51 (the usual ΔT correction factors).
Quick start sizes: typical UK rooms with a 2.4 m ceiling — change them to the real room.
Not a full heat loss calculation. Heat pumps need a proper room-by-room heat loss (MCS).
These results are a guide only. Check quantities with your supplier before ordering, and radiator sizes with the manufacturer or a heating engineer.
