A small slab fails in the same handful of places every time: a base that was not compacted, forms that moved, mix that was too wet, and a finish that was left to dry in the sun. The bag count is the easy part. This page is the sequence that the concrete calculator assumes you will follow when it tells you an 8×10 pad is bag territory.
It is not a substitute for a licensed contractor on anything structural, and it is not a story about fifty heroic pours. It is the order of operations that keeps a shed pad, an AC pad, or a small landing from cracking in the first winter.
What “small” means here
Small on this site is a pad under about 100 square feet — an 8×8, 8×10, or 8×12 at four inches. That is roughly 0.8–1.2 cubic yards with waste, which is bags and a mixer, not a truck. Once you cross a 10×10 the short-load fee starts arguing for ready-mix; once wheels are involved you are in a different job. See bags vs ready-mix and slab thickness before you treat a parking pad like a landing.
If the slab holds up a structure, sits on a property line, or needs a permit, stop and get the local answer. The rest of this page assumes a free-standing residential pad on reasonably level ground.
1. Layout and square
Drive four stakes at the corners and run string. Check square with the 3-4-5 method: 3 ft along one side, 4 ft along the other, 5 ft on the diagonal. On an 8×10 that scales to 6-8-10. Equal diagonals are the faster check once the strings are up.
Mark the outside of the forms, not the finished face, if you are using 1½″ lumber — otherwise the pad comes up an inch-and-a-half small in both directions and every bag you bought was sized for a slab that does not exist.
Slope matters even on a pad. ⅛ inch per foot away from the building is the usual residential fall. On an 8 ft run that is 1 inch. A dead-level pad against a house is a puddle against a house.
2. Excavate for the slab and the base
The hole is thicker than the concrete. A 4″ slab on a 4″ compacted gravel base is an 8″ excavation, plus whatever topsoil you have to strip to get to undisturbed soil. Organic soil left under a pad is a void waiting to happen.
Undisturbed soil is the floor. If you over-dig, put the extra back as compacted
gravel, not as loose dirt. The waste factor
on the concrete exists partly because the subgrade is never as flat as the
sketch. A ½-inch average dip on an 8×10 is 80 × (0.5 ÷ 12) = 3.3 ft³ — about
six extra 80 lb bags all by itself.
3. Compact the base
Four inches of compacted crushed stone (not rounded pea gravel — it will not lock) in two 2-inch lifts. Rent a plate compactor for the afternoon; a hand tamper on 80 square feet is how people convince themselves they compacted when they did not.
The base should finish dead to the bottom of the forms, not “close.” High spots become a 3½-inch slab, which is how a “4-inch” pour measures 3.5″ in places. That is a thickness problem, not a bag-count problem, and it is cheaper to rake the base than to live with it.
4. Build the forms so they cannot move
2×4s on edge are the usual form for a 4″ slab (a dressed 2×4 is 3½″, so you either accept a 3½″ pour or use 2×6s and pour to a 4″ mark). Stake every 2 feet, and every corner twice. Wet concrete is heavy — a cubic foot weighs about 140 lb after it is placed — and it will walk a lightly staked board outward in the middle of the run.
Coat the inside face with a release agent or a cheap oil so the boards come off cleanly the next day. Brace anything longer than 8 ft; an 8×12’s long side is already asking for a mid-span kick.
Check square and slope again after the stakes are in. This is the last cheap moment to fix either.
5. Reinforcement sits in the middle, or it is decoration
Welded wire mesh or a light rebar grid does not stop a pad cracking. It holds a crack tight so the two sides still act as one piece. On a pad this size, mesh on chairs (or dobies) at mid-depth is the usual call. Mesh laid on the gravel and “lifted as you pour” ends up at the bottom, which is the compression zone, which is where it does nothing.
The size pages print a mesh-sheet and rebar-stick takeoff for the footprint you picked. Use it. Overlap mesh a full 6 inches and tie it so it cannot drift when the first barrow lands on it.
6. Mix for a slab, not for a post hole
Standard 80 lb mix — QUIKRETE #1101 or the matching Sakrete high-strength bag — is what this site’s bag math assumes. Fast-setting mix is the wrong product for a pad. It is for posts. It gives you minutes, not an hour, and the yield is not the 0.60 ft³ figure on the calculator. If you want the brand comparison, see Quikrete vs Sakrete.
Water is the lever people pull when they should not. The bag lists a range; the low end of that range is a stiffer, stronger mix that you can still work. Extra water makes it easy to place and easy to dust, scale, and crack later. If the mix will not come down the chute of a rented mixer, the mixer is under-fed or you are adding bags faster than it can blend — not a reason to reach for the hose.
One 80 lb bag is 0.60 ft³. A typical drum mixer takes two bags per batch. An 8×10 at 4″ with 10% waste is on the order of 50 bags, which is 25 mixer batches. At five minutes a batch you are looking at two hours of mixing before you count placing and finishing. That is why the calculator also prints wheelbarrow loads. One person can do this. Two people do it better, because someone has to be placing while someone is mixing, or the first corner starts to set.
Have the whole bag count on site before you add water to the first batch. A mid-pour store run is how small slabs get a cold joint.
7. Place, screed, float, edge, broom
Start in the far corner and work toward the mixer so you are not walking on fresh concrete. Dump, spread with a come-along or hoe, and keep a wet edge — each batch should land against concrete that is still plastic.
Screed off the forms with a straight 2×4 in a sawing motion. Two people on an 8–12 ft run; one person on an 8 ft run if the board is stiff. Fill low spots immediately and screed again. This is the last chance to get the slope right.
Wait until the bleed water leaves the surface before you float. Floating too soon works water back into the top and you get a weak skin. A magnesium float is the usual DIY tool; wood floats grab more on a small pad and are fine. Edging comes next, once the surface will take the tool without crumbling — it rounds the perimeter so the arris does not spall the first time something hits it.
A broom finish is the right call on anything that will be walked on wet. Drag a stiff broom once, in one direction, after the float, while the surface still takes a texture but no longer bleeds. Over-brooming pulls up sand and looks worse, not better.
Do not trowel a pad to a steel-burnished shine outdoors. That surface is slick when wet and it is extra work that a shed pad does not pay back.
8. Joints the same day
Concrete will crack. Control joints are how you pick where. On a pad under about 10 ft in both directions the rule of thumb (joint spacing in feet ≈ 2–3 × thickness in inches) may not force a joint on you. Cut one anyway if the pad is against a house, if the climate freeze-thaws hard, or if you do not trust the subgrade. A tooled joint on an 8×10, mid-span each way or one way, is an hour of work that disappears under dirt; a random shrinkage crack is a line you look at for twenty years.
Cut depth is a quarter of the slab — 1″ on a 4″ pour. Tool it the same afternoon, or saw it the next day once the surface will carry a blade without ravelling. Waiting a week is waiting until the slab has already cracked somewhere else.
The size pages spell out panel counts for the larger footprints. A pad often has a panel count of one. That is a fact about the geometry, not permission to skip the question.
9. Cure it like you mean it
Concrete does not dry to get strong. It hydrates. If the water leaves in the first day — sun, wind, 90 °F air — hydration stops at the surface and you get dusting. Cover with plastic, wet burlap, or a curing compound within a few hours of finishing, and keep it damp for at least three days, a week if you can stand it.
Foot traffic after a day or two. Do not park anything on it, do not put the shed down, do not set the condenser, until you are past the early-strength window. The calculator hub’s curing table is the same rule this page uses: vehicles at seven days, design load at 28. A small pad is not exempt because it is small. It just fails in a smaller area.
Strip the forms the next day once the edges will hold. Backfill against the slab so water is not running under it.
What this page is not
It is not mix design, soil engineering, or a permit. Expansive clay, a high water table, a frost line that wants footings, or a pad that carries a wall are all reasons to hire the job out — the DIY vs hire numbers exist because a botched small pour is not actually cheap.
The quantity math — bags, yards, waste, joints, mesh — lives on the size pages and in how much concrete do I need?. Use those for the order. Use this page for the afternoon, in order, without skipping the base.