Cement Calculator — Bags and Yards
Small concrete jobs are bought by the bag, and the bag count is a volume problem wearing an apron. Enter your slab or footing’s dimensions, and this calculator returns its volume in cubic feet and yards — and the number of 80 lb or 60 lb premix bags that volume actually requires, before optimism rounds it down.
Bags Are Volumes, Not Weights — Read the Yield
The shelf at the builders' merchant invites a category error this calculator exists to prevent: bags are labelled by weight (80 lb, 60 lb) but consumed by volume, and the bridge between the two is the manufacturer's yield — an 80 lb bag of premix yields 0.60 cu ft of placed concrete, a 60 lb bag 0.45, a 40 lb bag 0.30. The yields are consistent because premix density is consistent (the mix is Portland cement, sand and gravel in the bag, needing only water), and they make the estimate a clean division: volume of the form ÷ yield per bag, rounded up, always — concrete is not sold in fractions of a bag and a form filled to 97% is a form betrayed. The same volume in cubic yards (÷ 27) rides along in the answer because the bag-versus-truck decision — the note's one-yard rule — is the second question every slab asks.
Worked Example: The 10 × 10 Shed Slab
The defaults pour the weekend classic: a 10 × 10 ft slab, 4 inches thick. Volume: 10 × 10 × (4/12) = 33.3 cu ft, which is 1.23 cu yd. In 80 lb bags: 33.3 ÷ 0.60 = 55.6, rounding up to 56 bags — and the note's two disciplines immediately engage: buy about 62 with the waste allowance, and notice that the job has crossed the one-yard line where a ready-mix truck (with its chute, its consistency, and its driver who has seen your forms before) usually wins against a day of mixing 56 bags by hand or wheelbarrow. In 60 lb bags the count is 75 — lighter lifts, more of them, the eternal trade the selector lays bare. The calculator's neutrality is the point: it prices the volume faithfully in every currency (feet, yards, each bag size) and lets the project's back, budget and timeline choose.
Thickness: The Dimension That Lies Quietly
Of the three inputs, thickness is the one that inflates estimates — and slabs — most quietly: forms set on unlevel subgrade deliver a 4-inch design at 5 inches in the dips, and every extra inch across a 10 × 10 slab is 8.3 more cubic feet, 14 more bags. The professional defences are grading and compacting the sub-base properly, screeding to honest forms, and — the calculator's contribution — pricing the slab at the thickness it will actually average (measure the form depth at several points, not at its proudest corner). Depth discipline matters structurally as well: a footing's depth below frost or a slab's section under load are engineering figures, and the estimator who runs this page twice — at the design thickness and at the as-built one — knows both the order and the contingency, which is the whole of estimating wisdom in one habit, learned usually from exactly one slab that came up short at four in the afternoon.
Curing: The Week After the Count
The bag count buys the pour; curing decides what the pour becomes. Concrete's strength is a chemical schedule, not a drying event — it gains most of its rating across the first week of moist curing and keeps improving for a month, while a slab left to bake dry on its first afternoon can forfeit a third of its design strength at the surface where it matters. The site habits are as cheap as they are decisive: keep the slab damp or covered for at least the first several days (the hose, the sprinkler, the plastic sheet), keep foot traffic off for a day and vehicles for a week, and respect cold and hot weather as schedule-warpers in opposite directions. None of this appears in the calculator's arithmetic — which is precisely this section's point: the number gets the material to the forms; everything after is stewardship.
Reinforcement and Joints: The Slab's Skeleton and Seams
Volume's companions on pour day deserve their paragraph before the tools list. A slab this size earns steel or fibre almost by reflex — welded wire mesh or a rebar grid on chairs at mid-depth in the 4-inch section, never lying on the subgrade where it armours nothing and never floated to the surface where it decorates rust — and control joints, tooled or sawn to a quarter of the thickness at spacings around 8–12 feet, which decide in advance where the slab will crack (beautifully, invisibly, along the seam) instead of letting it choose. The joints also edit the pour plan: each jointed panel is a natural unit for a bag-mixed job, letting a solo worker place and finish one square at a time without a cold joint down the middle of an unplanned panel. Price the mesh and the joints with the bags; the slab's long memory will thank the arithmetic, usually in the fifth winter when the unjointed neighbour's driveway has mapped every crack this one was spared.
Related Tools
More from the Construction Calculators hub: the Concrete Block Fill Calculator for grout volumes in block walls, and the Hole Volume Calculator for post holes — the bag's other natural habitat.
Frequently Asked Questions
How many 80 lb bags make a full cubic yard?
A cubic yard is 27 cu ft; at 0.60 cu ft per bag, that is 27 ÷ 0.60 = 45 bags — the figure behind the note's truck rule, and a memorable one: forty-five bags is roughly 3,600 lb of lifting, which is why the yard line is where backs vote for the truck. In 60 lb bags the yard takes 60; in 40 lb bags, 90. Anyone quoting a dramatically different count is using a different yield or a different yard, and the yields printed on the bag itself are the court of appeal.
How much water per bag, exactly?
Follow the bag's own instructions — typically about 2.6 quarts (roughly 2.5 litres) for an 80 lb bag, scaled by size — and resist the ancient temptation to add extra for workability: surplus water buys a few easy minutes of placing at the price of the cured strength, permanently. The mix should look like stiff oatmeal, hold a ridge, and never soup. If the day's heat or the haul across the site makes placement slow, the honest remedies are smaller batches, a retarder in commercial work, or the truck the note keeps recommending — not the hose.
Can I use this for footings and post holes, too?
Yes — any rectangular volume, and for round post holes the hole-volume page on this hub computes the cylinder (π r² × depth) whose cubic feet then divide by the same bag yields: a 12-inch-diameter, 3-foot-deep hole is about 2.36 cu ft, or four 80 lb bags, a figure the fence builders among the readership will recognise as an old friend. Footings price identically to slabs with their depth in the thickness box; stepped footings price as a sum of rectangles, one calculator run per step.
What strength is bagged premix, typically?
Standard premix is typically rated around 4,000 psi at 28 days when mixed and cured as directed — sufficient for slabs, footings and the general run of residential work — with high-strength and fast-setting variants on neighbouring shelves for their niches (posts that must stand the same afternoon, structural repairs). The rating assumes the water discipline of the previous answer and proper curing (kept moist for the first days, protected from freezing): the bag's number is a potential the site's habits redeem or squander, which is true of every material on this hub and worth saying at the bag's page in particular.
How long do bags keep, and how should I store them safely?
Bagged premix and cement products drink atmospheric moisture and pre-hydrate into lumps and lost strength — months, not years, is their shelf life, shorter in damp sheds. Store bags off the ground, under cover, wrapped once opened, and rotate stock first-in-first-out; a bag that has gone to gravel-textured lumps that do not crush between fingers is a bag to discard, not to dilute into a structural pour. Site discipline about storage is invisible in this calculator's arithmetic, and decisive in the slab it prices — a Pallet of fresh bags under a tarp is a foundation's first, cheapest quality-control step.
Bags or ready-mix — where exactly is the line drawn?
The note's one-yard rule is the trade's centre of gravity, with honest adjustments: ready-mix carries minimum-load and short-load fees that punish small orders (some plants will not roll for under a yard at any sane price), while bag mixing's costs are labour, time, consistency risk and the 45-bags-per-yard lifting census. Remote sites, tiny pours at odd hours, and jobs needing a bag-an-hour pace belong to bags; anything past a yard in one continuous placement belongs, back permitting, to the truck — and the calculator prices both currencies precisely so the line can be drawn per project rather than per habit.