How Much Concrete For A Sidewalk

Before you order a truck or stack bags in the driveway, you need a realistic quantity for how much concrete for a sidewalk. Too little concrete means a cold joint halfway through the pour, a second trip, and a weak seam; too much means bags or a short-load fee you did not need. This guide walks through a 30 ft sidewalk, 3.5 ft wide and 4 in thick with the volume formula, the typical bag yields from our material data, and a 10% waste allowance, and it ends with a range you can actually order against. Everything here is an estimate — measure twice, check the bag label, and buy a little extra for the parts of the job you cannot see until the forms come off.

What makes a 30 ft sidewalk, 3.5 ft wide and 4 in thick different from a plain slab is the shape: a sidewalk is a long, narrow slab, so the volume comes from length times width times depth, and the waste hides in the uneven base and in the joints between sections. That shape decides which formula applies and where the waste hides, so the worked example below does the arithmetic for this exact shape rather than quoting a generic per-square-foot figure. If your dimensions differ, the same steps still work — substitute your own length, width and depth, keep the 10% waste allowance, and round up to whole bags.

How the calculation works

The calculator needs the shape of the pour, the depth, and the bag size you plan to buy, and each one changes the answer. For a rectangular pour the volume is length times width times depth, all in feet, divided by 27 to get cubic yards — one cubic yard is 27 cubic feet (0.76 cubic metres). Depth is where most mistakes happen: 4 in (10 cm) is 0.333 ft, 6 in (15 cm) is 0.5 ft, so always convert inches to feet before multiplying. A slab that is 10 ft by 10 ft at 4 in thick is 100 times 0.333, or 33.3 cubic feet, which is about 1.2 cubic yards before waste. The same slab at 6 in thick is half as much again, about 1.9 cubic yards, which is why the thickness in the keyword matters as much as the footprint.

Once the volume is known, it is converted to bags. Our material data gives typical yields of about 45 80 lb (36.3 kg) bags per cubic yard and about 60 60 lb (27.2 kg) bags per cubic yard, because an 80 lb bag yields about 0.6 cubic feet and a 60 lb bag about 0.45 cubic feet — those are typical values, and the bag label on the exact product you buy governs. On top of that, add the typical waste allowance of about 10% for spillage, an uneven subgrade, and the concrete that stays in the mixer and the wheelbarrow. The worked example below shows the low end with no waste and the high end with the full 10% allowance, so you can see the range instead of a single number that pretends the pour will be perfect.

A sidewalk is a long narrow slab, so the depth is the number to check most carefully: a base that settles or a form that is set a little deep changes the volume along the whole run. At 3.5 ft wide this walk is a little narrower than the common 4 ft, which saves concrete but feels tighter to walk on — the width does not change the method, only the arithmetic. If the sidewalk has a curve, measure it in straight sections and add them; a curved section measured as one long rectangle will overstate the volume.

Worked example: a 30 ft sidewalk, 3.5 ft wide and 4 in thick

Here is the full arithmetic for a 30 ft (9.1 m) sidewalk, 3.5 ft (1.1 m) wide and 4 in (10 cm) thick.

Step 1 — convert to feet. Length 30 ft. Width 3.5 ft. Depth 4 in ÷ 12 = 0.333 ft.

Step 2 — volume in cubic feet. 30 × 3.5 × 0.333 = 35.0 cubic feet.

Step 3 — convert to cubic yards. 35.0 ÷ 27 = 1.30 cubic yards, or about 1.3 cubic yards.

Step 4 — bags, low end. At the typical yield of about 45 80 lb bags per cubic yard: 1.30 × 45 = 58.3 bags, rounded up to 59 bags. With 60 lb bags: 1.30 × 60 = 77.8, rounded up to 78 bags.

Step 5 — bags, high end with the 10% waste allowance. 1.30 × 1.10 = 1.43 cubic yards, or 65 80 lb bags (86 60 lb bags). Order 59–65 80 lb bags and keep one or two spare; a sidewalk that runs short at the far end leaves a visible patch that will always show.

Cost guidance

Costs on this page are ranges, labelled typical retail range — check the current shelf price before budgeting. No single national price is quoted, because bagged concrete, ready-mix and delivery charges vary by region, retailer and supplier, and a price that is right in one town can be wrong in the next. The way to budget is to multiply, not to guess: take the quantity from the worked example (1.3–1.4 cubic yards, or about 59–65 80 lb bags), multiply it by the shelf price per bag or the per-cubic-yard price your local supplier quotes, then add delivery, formwork timber, rebar or mesh, and a finishing tool if you do not own one. For a small pour like this one, bagged mix bought by the bag is usually the practical route; once the volume passes roughly 1–2 cubic yards, a ready-mix short load often works out cheaper per yard even after the delivery charge, so price both options against the same quantity.

If the job grows while you measure — a footing that turns out longer than the plan, a sidewalk that needs an extra section — redo the volume from the new dimensions rather than guessing a percentage on top of the old total. Concrete quantities scale with volume, not with length alone, so a small change in depth changes the total more than most people expect. And if you are comparing bagged mix with ready-mix, remember the comparison only works when both quotes cover the same volume including waste, the same delivery distance, and the same extras; a cheap per-bag price with an expensive delivery is not a cheap job.

Step by step: measure, calculate, order

1. Measure twice. Measure the sidewalk length in feet, the width in feet, and the depth in inches at several places along the run. Measure each run separately — a footing that turns a corner is two runs, not one — and write every dimension down in feet and inches before you convert anything.

2. Convert and calculate. Convert inches to feet (divide by 12), then apply the volume formula for your shape: length times width times depth divided by 27 for a rectangle, or pi times radius squared times depth for a round tube or post hole. Use the calculator on this page to check your arithmetic, and run it twice with the low and high depth if your excavation is uneven.

3. Add waste and round up. Add the typical 10% waste allowance for spillage and an uneven subgrade, then round up to whole bags or to the nearest quarter cubic yard for a truck. A bag cannot be bought in fractions, and a pour that stops short is far more expensive than one spare bag.

4. Order and check the delivery. Order the high end of the bag range below, all the same mix, all the same product and mix, and check the bag labels against the yields the calculator assumed. If you are ordering ready-mix, confirm the slump, the mix strength and the delivery window, and make sure the truck can reach the pour.

5. Before you pour. Set and brace the forms, check the depth with a straightedge in several places, wet the subgrade lightly so it does not steal water from the mix, and have your screed, float and curing cover within reach. Concrete waits for nobody once it is mixed.

Common mistakes to avoid

Three mistakes ruin more small concrete jobs than anything else. First, using the nominal depth instead of the real one — a slab formed with 2x4s is often poured a little deeper than 4 in once the base settles, so measure after the base is compacted. Second, mixing bag counts from two different bag sizes: 80 lb and 60 lb bags yield different volumes, so pick one size and do all the arithmetic in that size. Third, pouring without a plan for finishing and curing — concrete that dries too fast cracks, and concrete that is not screeded level cannot be fixed later. Have the tools, the help and the curing cover ready before the first bag is mixed, and never add extra water to make a stiff mix easier to place; it weakens the finished pour.

Frequently asked questions

How much concrete do I need for a 30 foot sidewalk?

Multiply length times width times depth and convert to cubic yards. A 30 ft sidewalk, 3.5 ft wide and 4 in thick, is about 1.3 cubic yards — 59–65 80 lb bags including the typical 10% waste allowance. Measure the depth in several places, because a base that is deeper at one end needs more than the drawing says.

How many bags of concrete for a sidewalk?

Divide the volume by the bag yield and add about 10% waste. For this 30 ft walk the answer is 59–65 80 lb bags or 78–86 60 lb bags. A longer or wider walk scales directly with the area, so a walk twice as long needs roughly twice the bags at the same thickness.

What base do I need under a sidewalk?

A compacted gravel base, usually about 4 in of compacted crushed stone, gives the slab something stable to sit on and helps drainage. The base is not part of the concrete volume — it sits underneath it — but a base that is not compacted will settle, and the slab will crack as it follows the base down. Compact in layers and check the level before you set the forms.

How long before I can walk on a new sidewalk?

In normal conditions, about 24–48 hours for foot traffic, but the concrete keeps gaining strength for weeks. Keep it damp or covered for the first few days, keep vehicles off it for at least a week, and do not let de-icing salts sit on a new slab in its first winter — fresh concrete is at its most vulnerable in the first season.

Should a sidewalk slope for drainage?

Yes — a sidewalk should slope slightly away from the house and toward the street or a drain, usually about a quarter inch per foot. The slope does not change the volume estimate in any meaningful way, but it changes the formwork: set the forms to the slope before you calculate the depth, and measure the depth on the high side so the low side is not thinner than planned.

Related calculators

Results are estimates — verify with the product label. Coverage, pack sizes, yields and prices vary by brand, product line, surface and region; the label on the product you buy governs. Cost figures are typical retail ranges — check the current shelf price before budgeting.