About

Foundations are the anchor of any structure, and their material requirements must be calculated with high precision. This tool is engineered for strip foundations and continuous footings, calculating not just the concrete volume, but the specific steel reinforcement (rebar) needs. It accounts for the mandatory overlaps (splices) required when joining bars, which typically adds 10% to 15% to the total steel length.

The calculator also estimates the surface area of formwork required to contain the wet concrete. By cross-referencing standard rebar sizes (Imperial #3, #4, #5 or Metric 10M, 15M), it provides a total weight estimate, essential for logistics and delivery planning. It includes a basic check against standard frost line depths to alert users if the footing might be too shallow for freezing climates.

Formulas

Concrete volume (V) is calculated by the linear length (L) multiplied by the cross-sectional area (W × D).

V = L × W × D

Rebar calculation determines total linear footage (Rtotal) based on the number of continuous bars (nb) and the splice factor. A standard overlap is 40 times the bar diameter (d).

Rtotal = (L × nb) + SpliceAllowance

Reference Data

Rebar Size (US)	Diameter (in)	Weight (lb/ft)	Metric Equiv	Standard Splicing (40d)
#3	0.375	0.376	10M	15 in
#4	0.500	0.668	13M	20 in
#5	0.625	1.043	16M	25 in
#6	0.750	1.502	19M	30 in

Frequently Asked Questions

Rebar is typically sold in 20ft or 40ft lengths. When a foundation wall exceeds this length, bars must be joined. Simply butting them together provides no structural continuity. They must overlap by at least 40 times their diameter to transfer tensile loads effectively.

Formwork area is the total surface area of the timber or panels that touch the wet concrete. For a strip footing, this is calculated as 2 × Height × Length (for both sides of the wall/footing).

No. This tool calculates material quantities based on your inputs. It does not analyze soil bearing capacity, load paths, or seismic requirements. Always consult local building codes.