How to use this calculator
- Enter the internal size of the tank and the freeboard.
- Enter wall, base and roof slab thickness.
- Pick the concrete mix and steel rate.
- Read cement bags, sand, aggregate, steel and shuttering.
How do you calculate materials for an RCC water tank?
Walls = [(L + 2t)(W + 2t) − LW] × (depth + freeboard)
Base and roof = (L + 2t)(W + 2t) × slab thickness
Dry volume = wet volume × 1.54
Cement bags = dry volume × (cement part ÷ sum of parts) × 1,440 ÷ 50
1.54 allows for voids and bulking. Cement bulk density is 1,440 kg/m³ and a bag is 50 kg (0.0347 m³).
Worked example
With internal length 3 m, internal width 2 m, water depth 1.5 m, freeboard 0.3 m, wall thickness 200 mm, base slab thickness 200 mm, roof slab thickness 150 mm, concrete mix: M20 (1 : 1.5 : 3), steel 100 kg per m³ of concrete, wastage 5 %:
Cement: 58 bags (50 kg)
The calculator also gives tank capacity 9,000 litres; concrete (wet, incl. wastage) 7.08 m³; sand 2.97 m³.
Assumptions and limits
- Rectangular tank, walls on the base slab, no haunches, manholes or columns.
- Nominal mixes; design mixes for M25 and above use less cement per m³.
Frequently asked questions
How many cement bags are needed for 1 m³ of M20 concrete?
About 8 bags. Dry volume is 1.54 m³; cement is 1 part in 5.5, which is 0.28 m³ or 403 kg.
What thickness should an RCC water tank wall be?
Small domestic tanks often use 150 to 200 mm walls. Larger or underground tanks need 200 to 300 mm or more, set by structural design under IS 3370.
How do you convert tank size to litres?
Multiply internal length × width × water depth in metres, then multiply by 1,000. A 3 × 2 × 1.5 m tank holds 9,000 litres.
References
- BIS IS 456:2000, Plain and Reinforced Concrete
- BIS IS 3370 (Parts 1 and 2):2009, Concrete structures for retaining aqueous liquids
- CPWD, Analysis of Rates