Raft foundation rebar calculator
This calculator estimates a raft: reinforced concrete = length × width × thickness; steel = that volume × a typical ratio of 100 kg per m³; cement = volume × 350 kg ÷ 50 kg per bag. It also gives the plain blinding under the raft from its thickness and projection, with cement at 250 kg per m³.
Calculator
- Reinforcing steel
- 24,000 kg (24 t)
- Reinforced concrete
- 240 m³
- Plain blinding concrete
- 30.7 m³
- Cement, reinforced concrete
- 1,680 bags
- Cement, blinding
- 154 bags
Method, step by step
- Multiply the raft length by its width and thickness in metres for the reinforced concrete.
- Steel weight = reinforced concrete × the steel ratio in kg per m³.
- Cement = volume × cement content ÷ bag weight, rounded up.
- Blinding = (length + twice the projection) × (width + twice the projection) × its thickness; its cement is worked out the same way with its own cement content.
Worked example
Inputs
- Length (m): 20
- Width (m): 15
- Raft thickness (m): 0.8
- Blinding thickness (m): 0.1
- Blinding projection per side (m): 0.1
- Reinforced concrete = length × width × thickness = 20 × 15 × 0.8 = 240 m³.
- Steel weight = 240 × 100 = 24,000 kg (24 t).
- Cement for the reinforced concrete = 240 × 350 ÷ 50 = 1,680 bags, rounded up.
- Blinding = (20 + 2 × 0.1) × (15 + 2 × 0.1) × 0.1 = 20.2 × 15.2 × 0.1 = 30.7 m³.
- Cement for the blinding = 30.7 × 250 ÷ 50 = 154 bags, rounded up.
Result
- Reinforcing steel
- 24,000 kg (24 t)
- Reinforced concrete
- 240 m³
- Plain blinding concrete
- 30.7 m³
- Cement, reinforced concrete
- 1,680 bags
- Cement, blinding
- 154 bags
Assumptions and default ratios
| Ratio | Default value |
|---|---|
| Raft steel ratio (typical) | 100 kg per m³ of concrete |
| Cement in concrete | 350 kg/m³ |
| Cement in plain blinding concrete | 250 kg/m³ |
| Cement bag weight | 50 kg/bag |
- The steel ratio of 100 kg per m³ and the cement contents are editable typical values for preliminary estimates, not design values; the structural drawings, the bar bending schedule and the approved mix govern.
- The raft is rectangular with a constant thickness. Thickenings under columns or walls, excavation, backfill, waterproofing and waste are not included.
- The projection and thicknesses in the example are illustrative; take them from the foundation drawing, and enter 0 for the projection if the blinding matches the raft.
Frequently asked questions
How many kg of steel are there in one m³ of raft?
The default is 100 kg per m³, a typical ratio for preliminary estimates. The real ratio depends on the raft thickness, the building loads and the soil; adjust it to your drawings.
How many cement bags does a 240 m³ raft need?
At 350 kg of cement per m³ and 50 kg bags: 240 × 350 ÷ 50 = 1,680 bags. Change the cement content if your approved mix differs.
Why is the blinding larger than the raft?
When the drawing shows the blinding projecting beyond the raft, its length and width grow by twice the projection. Enter the projection from the foundation drawing, or 0 if there is none.
Does this replace the bar bending schedule?
No. The result is for early pricing and comparison; steel orders and payment certificates rely on the actual lengths in the schedule.
Add the result to your project BOQ in Ta3mir
In Ta3mir the same calculators work inside your project: send a result to a BOQ line in one click and save your own default ratios for every calculation.