Isolated Footing (Pad) Calculator — Concrete, Rebar Grid & Chairs

Size materials for a single pad footing. Enter plan dimensions (length × width), thickness, waste %, and choose a rebar grid (spacing, bar sizes, cover, stock length, laps). Optionally include a top mat and estimate chair/support counts. Results include concrete volume (m³, yd³, ft³), bag counts, rebar lengths/weights, chair counts, and a grouped cut list.

Units Metric (m, mm) US Customary (ft, in)

Inputs

Footing length (L)

Footing width (B)

Thickness (D)

Waste allowance

Covers spillage and variation (many jobs use 4–10%).

Rebar grid (bottom mat, optional top mat)

Concrete cover to bars

Typical ≥75 mm (3 in) for footings cast against earth.

Grid spacing (center-to-center)

Applies to both directions; we’ll enforce a minimum of 2 bars each way.

Bottom mat bar size

Top mat

Top mat bar size

Stock bar length

Common stock ≈6 m (20 ft); some suppliers carry 12 m (40 ft).

Lap splice settings (if bar length exceeds stock)

Lap = × db Min: m

Default 40 db (min 300 mm / 12 in). Edit per your code/engineer.

Optional end development (each end)

Set >0 if your detail needs extra embed beyond clear span to cover.

Chair / support estimate (per mat)

Chair spacing along length

Chair spacing along width

Typical support grids range ~0.6–1.2 m depending on bar size & load.

Bag yields (optional display)

US bag yields (QUIKRETE® Concrete Mix)

40 lb:ft³ 50 lb:ft³ 60 lb:ft³ 80 lb:ft³

Edit to match your brand; defaults from common data sheets.

Metric bag yields

20 kg:m³ 25 kg:m³ 40 kg:m³

Results

Concrete

Plan size: —

Thickness: —

Net volume: 0.000 m³

With waste: 0.000 m³

0.000 yd³ • 0.00 ft³

Bag counts (from total with waste)

40 lb: 0 • 50 lb: 0

60 lb: 0 • 80 lb: 0

20 kg: 0 • 25 kg: 0 • 40 kg: 0

Rebar (summary)

Bars each way (bottom): —

Bars each way (top): —

Total steel length: —

Estimated weight: —

Chairs / supports

Per mat: 0

Total (all mats): 0

Assumes rectangular grid of supports.

Rebar Totals

Bottom mat

X-dir bars (parallel to length): 0 × —

Y-dir bars (parallel to width): 0 × —

Segments per bar (max): 0 • Splices/bar: 0

Top mat

X-dir bars: 0 × —

Y-dir bars: 0 × —

Segments per bar (max): 0 • Splices/bar: 0

Cut List (grouped lengths)

Mat	Dir	Bar size	Qty	Length	Note

We group identical lengths; “remainder” is the last piece when stock lengths are used with laps.

How to use this footing (pad) calculator

Select metric or US units.
Enter length, width, and thickness; set your waste %.
Choose cover, grid spacing, bar sizes, stock length, lap settings, and optional end development.
Choose whether a top mat is required.
Optionally set chair spacing to estimate support counts.
Review concrete, bags, rebar totals/weights, chairs, and the grouped cut list. Round up for ordering.

Formulas & assumptions

Concrete volume: V = L × B × D; with waste V_w = V × (1 + waste%).
Clear dimensions for bars: L_clr = L − 2×cover, B_clr = B − 2×cover.
Bars each way: for spacing s, n(L) = max(2, ⌊L_clr / s⌋ + 1), n(B) = max(2, ⌊B_clr / s⌋ + 1).
Per-bar length: X-dir bars use L_req,X = L_clr + 2×dev_end; Y-dir bars use L_req,Y = B_clr + 2×dev_end.
Lap splice when L_req > L_stock: L_lap = max(lapDb × d_b, lapMin); effective advance per additional segment = L_stock − L_lap; k = 1 if single piece; otherwise k = ⌈(L_req − L_stock)/(L_stock − L_lap)⌉ + 1; splices/bar = k − 1.
Total steel (per mat, per direction): L_tot = n × ( L_req + (k − 1) × L_lap ). Weight uses kg/m ≈ d_mm² / 162 (metric) or standard lb/ft per US bar size.
Chairs/supports (per mat): rectangular grid count = ⌈L / s_x⌉ × ⌈B / s_y⌉.
Scope: Estimating tool—does not size the footing for bearing, shear, or punching, and does not design reinforcement. Follow your engineer’s drawings and local code (e.g., ACI 318 / Eurocode 2).

References

Concrete cover for reinforcement in footings cast against earth: ≥3 in (≈75 mm). See ACI 318 “Cover for reinforcement”.
Lap splices are development-length–based; many specs use 40 db minimum 300 mm/12 in as a starting point (verify per project). See TMS/ACI guidance.
Rebar mass: metric kg/m ≈ d²/162; US bar weights (lb/ft) per standard bar tables (CRSI).
Typical chair/support spacing ranges ~0.6–1.2 m; see CRSI Bar Supports recommendations.
US bag yields: QUIKRETE® Concrete Mix 1101 — 40/50/60/80 lb ≈ 0.30/0.375/0.45/0.60 ft³ per bag (product sheet).

FAQs

Do I need a top mat?

Many pad footings have only a bottom mat; a top mat may be required for punching shear, heavy biaxial moments, or seismic detailing—follow your engineer’s drawings.

How precise are chair counts?

They’re planning estimates using a rectangular support grid. Actual chair layout depends on bar size, loads during placement, and your bar support type.

Does this account for column pedestals?

No—this is footing concrete only. Add pedestal volume separately or ask for a variant with a pedestal input.