Concrete: ready-mix or bags by volume
By Srikanta M · Updated 2026-09-26 · 7 min read · Formula v1.0.0 verified 2026-09-10
To 0.5 yd³ (about 22 eighty-pound bags) site mixing wins; 1–2 yd³ compares bags against a short load; over 2 yd³ orders ready-mix. A 13×10 ft slab at 6 in is 2.4 yd³ net and 2.6 with 7%: order 3 yd³.
Where is the breakpoint in cubic yards?
An 80 lb bag yields about 0.6 ft³: a yard needs about 45 bags. Past 20 bags mixing eats half a day and consistency drops.
Plants charge short-load fees and quarter-yard increments: 0.5 yd³ can bill like 1 yd³. The 0.5–1 yd³ band always compares case by case.
Ready-mix or bags by volume (guide)
| Volume | 80 lb bags | Call |
|---|---|---|
| Under 0.5 yd³ | to 22 | Bags (mix on site) |
| 0.5–1 yd³ | 22–45 | Compare: bags vs short load |
| 1–2 yd³ | 45–90 | Compare bags vs small ready-mix |
| Over 2 yd³ | over 90 | Ready-mix (order extra, pump when needed) |
How is each shape cubed?
Slab is length × width × thickness. Strip footing is length × width × depth. Round column is πr²h. Solid stairs add steps as triangular prisms.
This guide estimates quantities. Depth, rebar and strength are signed by a qualified professional: this page never sizes structure.
slab V_ft³ = l × w × t ÷ 27 = yd³; order = V × (1 + 0.07), round up
What gets added besides volume?
Pumping when the truck cannot reach, formwork, rebar and blinding in trenches. None changes yd³, all change price.
In a trench, the strip footing covers volume (length × width × depth). Blinding, when used, quotes as its own 2–4 in line.
- Pumping: fixed line when direct discharge cannot reach.
- Formwork and rebar: separate lines, never inside yd³.
- Short-load fee: ask the plant minimum before cubing 0.5 yd³.
How do you order from the plant without error?
Order the 7% volume rounded up: 2.6 yd³ orders as 2.75 or 3 yd³. State design strength and slump, time and pump access.
Compare price per yard placed, not at the plant: haul and standby move the ranking between two plants.
Worked example: Slab 13×10 ft, 6 in thick
- V = 13 × 10 × 0.5 ft = 65 ft³ = 2.4 yd³
- At 7%: 2.4 × 1.07 = 2.6 yd³
- Round up: 3 yd³
→ Order 3 yd³ ready-mix (about 135 eighty-pound bags would be an unworkable site mix).
Common mistakes
- Ordering exact yards (2.6) with no rounding: plants batch quarters and the chute keeps 0.1 yd³.
- Mixing yd³ with sq ft: 215 sq ft of slab at 6 in is 4 yd³, not 215.
- Bag-mixing 2 yd³ to save the haul fee: 90 bags are 3.6 tons moved by hand.
From calculation to quote
Add labor, overhead and your margin, and generate the professional PDF.
FAQs
- How many 80 lb bags make a yard?
- About 45 bags (0.6 ft³ each). Past 1 yd³ price ready-mix even when mixing stays possible.
- Does the calculator include rebar?
- No: it cubes concrete only. Rebar, depth and strength follow the structural design.
- Does it handle stairs and ramps?
- Yes: stairs cube steps (tread × riser ÷ 2 × width × count) and ramps cube the wedge (length × rise ÷ 2 × width). Support slabs cube separately.
- Pump or direct discharge?
- When the truck cannot reach, add pumping as a line. Volume stays put, price moves (fixed service fee).
- How do I compare bags vs ready-mix in dollars?
- Multiply your bags (45 per yard) by bag price against short-load plus haul minimum. Without your yard prices no comparison holds.
Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.