How it is calculated
The concrete volume is the strip length along all walls × width × height plus a reserve. Longitudinal bars run the full length with 10 % added for laps and corners. Links are placed at the chosen spacing; each is the perimeter of the cage with 5 cm cover plus 20 cm for hooks. Rebar weight uses the standard 0.00617 × diameter² kg per metre.
Concrete = length × width × height × (1 + reserve)Longitudinal bars = length × number of bars × 1.1Weight of 1 m of rebar = 0.00617 × d² (d in mm)Example
A 36 m strip, 40 × 100 cm in section, needs 15.12 m³ of concrete with a 5 % reserve. Four Ø12 mm bars come to 158 m (141 kg, 27 bars of 6 m). 91 Ø8 mm links of 2.6 m add 237 m (93 kg). Formwork is 28.8 m² and the sand bed 5.2 m³.
Tips from practice
- Foundation depth depends on the soil and frost depth. For a house, follow the design or a geotechnical engineer.
- Bars need at least 40–50 mm of concrete on every side; use spacers.
- Pour the whole strip in one day without cold joints and compact it with a vibrator.
Questions and answers
What rebar does a strip foundation need?
For a house, typically 4–6 longitudinal B500 bars of 12–14 mm and 8 mm links at 300–400 mm.
How much concrete does a strip foundation need?
Multiply the strip length along all walls by its width and height. A 36 m strip of 0.4 × 1 m is 14.4 m³ before reserve.
Do I need a sand bed?
Yes. A 150–300 mm layer of sand or sand-gravel compacted in layers levels the base and reduces frost heave.
Found a mistake in a formula? Write to contact@planicalc.com