How Much Self Leveling Underlayment Do You Need
Three numbers decide the order, and only one of them is printed on anything. The other two are properties of the slab you are standing on, and getting the depth wrong by a sixteenth of an inch moves a garage floor by ten bags.
Bags come from area × average depth ÷ bag yield, rounded up. An 800 ft² garage at a quarter inch average is 16.67 ft³ of underlayment, which is 41 bags of a product yielding 0.45 ft³, with 10% waste. The number that decides all of it is the average depth, and it is the one nobody publishes: at five sixteenths that same floor takes 51 bags, and at three sixteenths it takes 31. Work yours out in the self leveling underlayment calculator.
The three numbers
Area, average depth, and the volume one bag yields. Multiply the first two to get the volume of underlayment the floor needs, add a waste allowance for what stays in the barrel and runs into joints, divide by the yield, and round up, because nobody sells most of a bag.
Only the third of those is printed on anything. Area you measure once and it does not argue with you. The average depth is the difficult one, and it is difficult for a reason worth being clear about: it is not a specification anybody can give you, because it describes how far out of level this particular slab is. No data sheet has it, no supplier can tell you, and it changes across the floor.
Measuring an average depth
Set a reference across the pour, a rotating laser or a long straightedge on packers, and take readings on a grid. Then average them. The grid is the part that does the work.
Readings taken wherever your eye is drawn come out biased deep, because dips are what catch the eye, and you over order. Readings taken across the flat middle of a floor come out biased shallow, and the job stops. Neither mistake announces itself, because both produce a number that looks just as confident as a correct one.
On a slab with a clear fall in one direction, a line of evenly spaced readings running across the fall is usually enough. On a slab that is simply wavy, use a grid and take more readings than feels necessary. The cost of an extra ten minutes with a laser is ten minutes.
What a sixteenth of an inch is worth
Bags scale with depth exactly. There is no curve in it and no threshold, so an error in the average is an error of the same proportion in the order. On a quarter inch pour, a sixteenth of an inch is 25%.
The same 800 ft² garage, the same product yielding 0.45 ft³ a bag, the same 10% waste, and only the average depth moving:
3/16 in average
- 31 bags
- Ten bags fewer than the quarter inch case
- What you order if you read the flat parts
1/4 in average
- 41 bags
- 16.67 ft³ poured, plus waste
- One bag covers 21.6 ft² at this depth
5/16 in average
- 51 bags
- Ten bags more than the quarter inch case
- What you order if you read the dips
Twenty bags separate the two ends of that, on one sixteenth of an inch of judgement either side. That is the single largest source of a short order on this job, and a calculator that returns one number hides it completely. Ours prints all three.
Run your own floor
Area, average depth and the bag yield off your own product. Returns bags to order with waste included, what one bag covers at your depth, primer volume, and the order at a sixteenth either side of your average.
Returns 41 bags, 18.33 ft³ to buy for, and 31 or 51 bags at a sixteenth either side
Open the live calculator →A coverage figure needs a depth attached to it
Coverage is yield divided by depth, and nothing else. So a coverage figure printed without the depth it was measured at is not a specification, it is a number.
Take a bag holding half a cubic foot. It covers 24 ft² at a quarter inch, 48 ft² at an eighth, and 12 ft² at a half. All three statements are true of the same bag at the same time. Read one of them off a sheet, apply it at a depth the sheet was not talking about, and the order is wrong by exactly the ratio of the two depths.
That is the mechanism by which somebody orders half of what a floor needs and finds out after the mixing has started. Data sheets state the yield in both forms, so the calculator accepts either, and when you give it a coverage figure it insists on the depth that goes with it.
It is the same trap as an epoxy coverage rate quoted without a film thickness, and it has the same fix: carry the condition with the number, every time. The coating version of this problem is worth reading alongside it.
Primer is not optional
Over bare concrete, prime. There are two separate reasons and only one of them is about adhesion.
A dry slab pulls water out of the mix while the material is still trying to flow. That kills the flow early, so the underlayment sets in roughly the shape it was poured in rather than flat, and air that was on its way out gets trapped as pinholes. The floor you were trying to level is now a level-ish floor with holes in it.
The second reason is bond. Underlayment poured onto a dusty slab bonds to the dust, and dust is not attached to anything. Primer locks that layer down and gives the pour concrete to hold.
Rates change with dilution and with how porous the slab is, so the number comes off the primer’s own data sheet. Put yours into the optional field and the calculator sizes it over the same area: at 300 ft²/gal, that 800 ft² garage takes 2.67 gal.
Fill the cracks first
Underlayment is designed to flow, which means it will find every crack, joint and pipe penetration on the floor and go into it. That costs you twice.
It comes out of the bag count without ever appearing in the depth you measured, so the order runs short for a reason nobody can see on site. And a crack filled with underlayment rather than with a filler made for the job is a crack that will telegraph back through the finished floor within a season.
Deal with them before you measure anything. The crack repair calculator sizes the filler separately, which is how it should be sized.
Minimum and maximum depth are product limits
Both ends of the range are real and both are specific to what you bought. Every product has a minimum below which it will not flow out and cure properly, and some are formulated to feather to nothing at the edge while others are explicitly not. Pouring a non feathering product thin at the perimeter reliably produces a crumbling edge.
There is a maximum per lift too, above which the material can crack, curl, or fail to cure through. Deeper than that and the answer is more than one lift, or a different product.
Those figures are on the data sheet for the product in front of you, and this site does not print a number for them. Inventing a thickness limit for somebody else’s product is the sort of claim our methodology exists to rule out.
Underlayment or self leveling epoxy
They get confused because both level themselves, but they are different materials doing different jobs at different points in the build.
An underlayment is a cementitious powder mixed with water and poured to correct the level of a slab before anything else happens to it. It is sold in bags and sized by volume, which is everything above.
A self leveling epoxy is a resin coating that goes on afterwards, once the slab is flat and dry. It is sold in kits, sized by coverage rate, and it flows out over a few millimetres rather than correcting a floor. The epoxy coverage calculator sizes that one.
Frequently Asked Questions
What are the three numbers?
Area, average depth, and the volume one bag yields. Multiply the first two to get a volume of underlayment, add a waste allowance, divide by the bag yield, and round up, because nobody sells most of a bag.
Only the third is published. Area you measure, and average depth you have to work out, which is the whole difficulty: it is not a specification anybody can give you, because it describes how far out of level this particular slab is.
How do I measure an average depth?
Set a reference across the pour, a laser or a long straightedge on packers, and take readings on a grid rather than wherever your eye is drawn. Then average them.
The grid is the part that matters. Readings taken at the dips that caught your attention come out biased deep and you over order; readings taken across the flat middle come out biased shallow and the job stops. Neither error announces itself, because both produce a confident looking number.
On a floor with a clear fall in one direction, a diagonal line of readings at even spacing across the fall is usually enough. On a slab that is simply wavy, use a grid and take more readings than feels necessary.
What is a sixteenth of an inch worth?
On a quarter inch pour, about a quarter of the order.
Bags scale with depth exactly, with no curve and no threshold, so an average that is out by a sixteenth is out by 25% when the target is a quarter inch. An 800 ft² garage at a quarter inch takes 41 bags of a product yielding 0.45 ft³. The same floor at five sixteenths takes 51 bags, and at three sixteenths it takes 31 bags.
That is the single largest source of a short order on this job, and it is invisible if the tool only gives you one number. The calculator prints all three.
Why does a coverage figure need a depth attached to it?
Because coverage is yield divided by depth and nothing else, so a coverage figure without a depth is not a specification.
A bag holding half a cubic foot covers 24 ft² at a quarter inch, 48 ft² at an eighth, and 12 ft² at a half. All three are true of the same bag. Read one of those off a sheet, apply it at a different depth, and the order is out by exactly the ratio of the depths, which is how somebody orders half of what the floor needs and does not find out until the mixing has started.
This is the same trap as an epoxy coverage rate quoted without a film thickness, and it has the same fix: always carry the condition with the number.
Do I need to prime, and how much?
Over bare concrete, in practice always. Two reasons, and they are separate.
A dry slab pulls water out of the mix while it is still flowing. That kills the flow before the material has levelled, so you get an underlayment that has set in the shape of the trowel marks rather than flat, and pinholes where escaping air had nowhere to go.
The second reason is bond. Underlayment poured onto a dusty slab bonds to the dust. Primer locks that layer down and gives the pour something real to hold.
The rate is on the primer's own data sheet and it changes with dilution and with how porous the slab is, which is why the calculator asks for your rate rather than assuming one.
How thick can it go?
That is a product limit, and both ends of it are real.
Every product has a minimum below which it does not flow out or cure properly. Some are formulated to feather to nothing at the edge and some are explicitly not, and pouring a non feathering product thin at the perimeter is a reliable way to get a crumbling edge.
There is also a maximum per lift, above which it can crack, curl, or fail to cure through. Deeper than that and the answer is more than one lift, or a different product.
Both numbers are on the data sheet for what you bought. This site will not print a figure for them, because inventing a thickness limit for a third party's product is exactly the sort of claim our methodology forbids.
Should I fill the cracks first?
Yes, and the voids too. Underlayment is designed to flow, which means it will find every crack, joint and pipe penetration on the floor and disappear into it.
That matters twice. It comes out of your bag count without appearing in the depth you measured, so the order runs short for a reason nobody can see. And a crack filled with underlayment rather than with a filler made for the job is a crack that will telegraph back through the finished floor.
Deal with them first, size them separately, then measure the average depth of what is left.
Is a self leveling underlayment the same as a self leveling epoxy?
No. They are different materials doing different jobs at different points in the build.
An underlayment is a cementitious powder, mixed with water, poured to correct the level of a slab before anything else happens to it. It is sold in bags and sized by volume, which is what this guide and its calculator are about.
A self leveling epoxy is a resin coating that goes on afterwards. It is sold in kits, sized by coverage rate, and it levels itself over a few millimetres rather than correcting a slab. Sizing one is the Epoxy Coverage Calculator's job.