How much concrete to set fence or deck posts — the hole volume with the post and the gravel base taken out, per hole and across every post, with the bags rounded up one hole at a time so you don’t over-buy or come up short.
Units
Mode
Post
Post shape
Post diameter (in)
Post length (in)
Post width (in)
A nominal “4×4” post is actually about 3.5″, and a “6×6” about 5.5″ — enter the real measured size.
Hole
Hole shape
Hole diameter (in)
Hole length (in)
Hole width (in)
Hole depth (in)
Gravel base (in)
The gravel base drains water away from the post; its depth is taken off the concrete, not added to it. Set it to 0 if you aren’t using one.
Suggests a hole size below — you can still override every field.
Volume and bag math only, from standard site-estimation formulas. Hole depth, frost-line requirements, post setting and bracing are up to you and your local code. Bag yields are the standard 40/50/60/80 lb product yields; fast-setting mixes can yield differently, so check your bag. Treat this as an order estimate, not a structural specification.
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Mixing your own, ordering the gravel, or pouring more than posts?
The gravel drainage base under each post comes out in tons and yards in the Gravel Calculator. If you’d rather batch concrete from raw cement, sand and gravel than buy bags, the Concrete Mix Ratio Calculator proportions it by ratio. And for slabs, footings and walls beyond post holes, the Concrete Calculator covers the general volume and cost.
What this fence post concrete calculator does — and its lane
Setting a post is not the same shape problem as pouring a plain column. The hole is bigger than the post, the post sits inside it, and most people put a few inches of gravel in the bottom for drainage — so the concrete only fills what is left. A generic cylinder volume ignores all three of those, which is why a general concrete tool over-states a post job. This page is built for the post-in-hole geometry specifically: it subtracts the post, subtracts the gravel base, and counts the holes. For slabs, footings and walls, the concrete calculator is the right tool; for batching from raw materials, see the mix ratio calculator.
Hole minus post minus gravel, per hole
The concrete in one hole is the hole’s footprint times the concrete depth, less the footprint of the post. The concrete depth is the hole depth minus the gravel at the bottom, because that gravel is not getting filled with concrete. Do this once per hole, round up to whole bags, then multiply by the number of posts.
concrete depth = hole depth − gravel depth round area = (π ÷ 4) × diameter² · square area = length × width concrete per hole = (hole area − post area) × concrete depth total = concrete per hole × posts × (1 + waste÷100) bags per hole = ⌈ concrete per hole ÷ bag yield ⌉
Two details quietly trip people up. First, the post is real volume: a nominal 4×4 timber post is about 3.5″ square in reality, and a 6×6 is about 5.5″, so use the measured size, not the name on the label. Second, bags round up per hole, not on the grand total — you can’t pour a third of a bag in one hole and keep the rest, so each hole takes a whole number of bags. That is why the bag total here is usually a touch higher than dividing the combined volume by one bag yield, and it’s the honest number for what you’ll actually open on site.
Round or square, fence post or pier
Posts and holes can each be round or square in any combination. If you’re pouring a column with no post in it — a deck footing or pier formed with a cardboard tube form, sometimes just called a round form tube — switch to the filled form-tube mode and the calculator fills the whole tube, skipping the post subtraction, while still allowing a gravel base. Bag yields are the standard product figures shown below; note that rapid-set post mixes can yield slightly differently, so glance at your own bag.
Bag
Yield (ft³)
Rough guide per 4×4 in a 10″ hole
40 lb
0.30
about 3 per hole
50 lb
0.375
about 3 per hole
60 lb
0.45
about 2 per hole
80 lb
0.60
about 2 per hole
Hole size, frost line, and metric
As a starting point, a post hole is often about three times the post’s width across and roughly a third of the post’s height deep, and always below the local frost line so winter heave can’t lift it. The optional helper fills those rule-of-thumb numbers in from your post size, but they’re only a guide — corner and gate posts carry more load and usually go deeper and wider, and your code or frost depth has the final say. Work in inches or centimetres; the result is shown in cubic feet, cubic yards and cubic metres together.
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Frequently asked questions
How much concrete do I need per fence post?
Hole volume minus the post and the gravel base. For a round hole: (π÷4) × diameter² × (depth − gravel), less the post’s cross-section. A 10″ round hole 30″ deep with 6″ gravel and a 3.5″ post is about 0.95 ft³, roughly two 60 lb bags. Enter your own numbers above; it works per hole then multiplies by the post count.
Why subtract the post and the gravel base?
Both take up space concrete doesn’t fill, so ignoring them means over-buying. A 6″ gravel base removes a fifth of a 30″ hole’s depth, and the post itself is real volume too. Leaving them out is the usual reason people end up with leftover bags. This tool subtracts both by default — set gravel to 0 if you’re not using a base.
Round bags up per hole or on the total?
Per hole. You can’t pour two-thirds of a bag in one hole and save the rest, so each hole rounds up to a whole bag. Rounding only on the grand total assumes you can split bags between holes and leaves you short. This calculator rounds up per hole, then multiplies — so the total is usually a little higher than a single division.
How deep and wide should a post hole be?
Rule of thumb: about 3× the post width across, and roughly ⅓ of the post’s above-ground height deep — and below your local frost line. A 6 ft fence usually means 2 ft deep or more. The helper fills these in from your post size, but corner and gate posts go deeper and wider, and your code wins.
Is a 4×4 post really 4 inches?
No — a planed 4×4 is about 3.5″, a 6×6 about 5.5″. The post’s real cross-section is what gets subtracted, so entering 4″ instead of 3.5″ would under-state the concrete a little. Use the measured size; for round or metal posts, measure the true outside diameter.
What about a form tube or pier with no post?
Use the filled form-tube / pier mode. We use the generic terms round form tube and cardboard tube form for the single-use cardboard cylinders sold for round columns and footings. That mode fills the whole tube with concrete and subtracts no post, since there isn’t one — you still get a gravel-base subtraction if you set one.