Grade Beam Calculator — ft, yd³, rebar
How to use this calculator
Enter the number of grade beams, the clear span, and the cross-section width and depth. Volume, weight, bag counts, and bar lengths feed straight into your concrete order and rebar takeoff.
Cost — pick ready-mix per yd³, 80-lb bags, or 60-lb bags.
Reinforcement — longitudinal bars (top + bottom) plus stirrups spaced per ACI 318-19. Lap splices at 40db, 1.5″ clear cover.
Labor — rate per linear foot, per yd³, or flat price.
Saved Calculations
| Time | Beams | Size | Vol yd³ | Concrete $ | Rebar lf | Rebar $ | Labor | Total |
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How to Use This Grade Beam Calculator
Set the number of beams and the clear span, then the cross-section width and depth. The diagram redraws as you type — tap any dimension label to jump to its input. Volume is computed net then multiplied by your concrete waste allowance (5% for clean straight runs, 10% typical, up to 15% on cut-up layouts with many pier intersections). Switch the results between yd³, ft³, and 60/80-lb bag counts to match how you are buying. Concrete weight uses 150 pcf (≈4,050 lb/yd³). For the piers or piles this beam spans between, pair it with the concrete pier calculator; for a continuous footing under a wall use the strip footing calculator.
Reinforcement per ACI 318-19
Grade beams carry tension from soil heave and span moments, so they are detailed like flexural members: continuous top and bottom longitudinal bars (#4–#6 here) with closed stirrups (#3 or #4) confining the cage. Maximum stirrup spacing is the lesser of d/2 and 24″ for non-seismic members (ACI 318-19 §9.7.6.2.2). Tension lap splices are taken at 40·db — a Class B approximation adequate for estimating — and clear cover is 1.5″ for bars not exposed to earth (use 3″ where cast against ground). Bar weights are #3 0.376, #4 0.668, #5 1.043, #6 1.502 lb/ft; stock length 20 ft. For a pile-supported version with a deeper cage see the pile cap calculator.
Formulas
Volume per beam = Width × Depth × Length. Total = Volume per beam × Number of beams × (1 + Waste%). Bag count = ⌈net ft³ ÷ 0.6⌉ for 80-lb bags or ÷ 0.45 for 60-lb. Longitudinal length = (Span + laps × 40·db) × bars × beams × (1 + rebar waste). Stirrup perimeter = 2 × (Width − 2·cover) + 2 × (Depth − 2·cover); stirrup count = ⌈Span ÷ spacing⌉ + 1 per beam. For slab-on-grade tied to these beams, the concrete slab calculator handles the flatwork and the concrete bag calculator sizes small bagged pours.
FAQ
What size grade beam do I need? For light residential loads 12″×24″ is typical; heavier loads or longer pier spacing use 18″×30″ or 24″×36″. Depth is usually governed by span and frost/expansive-soil cover, not the calculator — these defaults are estimating starting points only, so confirm size and reinforcement with the project structural engineer.
Bags or ready-mix? Bagged concrete is practical only for very small total volumes (a few yd³). At 0.022 yd³ per 80-lb bag a single 12″×24″×20 ft beam already needs roughly 60 bags, so ready-mix delivery is almost always cheaper and more consistent for grade beams. Compare unit prices with the spread footing calculator when isolated column footings are also in scope.
On pour day
Ready-mix in 2025 USA runs about $150–225/yd³ delivered, plus a $80–150 short-load fee under ~3 yd³ — four 12×24″×20 ft beams come to ~6 yd³ before waste, so a single beam alone often triggers the surcharge unless you combine pours with the slab. The #1 estimator mistake on grade beams is under-counting laps: bars over 20 ft need a 40·db splice (≈20″ for #5), and the calculator already adds one lap per bar but not the corner hooks at intersections — add ~5% extra LF for those. Keep 1.5″ clear cover on cage sides (3″ where cast directly against earth per ACI 318-19 §20.5.1.3) and place chairs at 4 ft o.c. so the bottom mat doesn't sag into the mud during the pour.
