Mortar Plaster Calculator | Render Quantities

I’m pretty sure everyone who has ever stood with a bag of cement in their hand about to plaster some sort of brick surface will know the feeling. There’s a certain sense of panic that takes hold as you realize you don’t know whether you’re going to be able to mix up enough mortar to complete the project. Guessing normaly involves either leaving half a bag behind, or dashing off to hardware mid-mix in order to top up supplies.

Fortunately for us all, the mortar plaster calculator above has done the maths for you. Simply enter in size of your walls and thickness of the plaster coat you want to apply. It’ll do the rest, turning a messy puzzle into a clear list of numbers so you can confidently purchase only what you need.

How to Use the Plaster Calculator

The obvious step is to understand the surface area. How long is it? How high is it? Then remove any doors, windows, vent etc. It seems tedious so a lot of people don’t do it, but remember that volume add up. If your render is thick, those hole add up to a fair amount of volume. This tool takes them away for you, calculating the net area which represents the plaster you’re going to use.

This is what most people mess up. They take gross area figure and wonder why there is leftover sand, or even worse, why they ran out before finishing the last corner.

But what about thickness? This is where presets in this calculator really shine through. Applying one skim coat inside your livivng room versus applying a three-coat stucco system on the outside wall is different than an animal entirely. With the inputs, you can specify exactly how thick each layer of stucco is (each has its own distinct structural purpose), down to the scratch, brown and finish coats. The scratch coat keys into masonry, the brown coat level the surface, while the finish coat give it final look. Underestimate thickness of the brown coat and no matter how well you finish with finishing trowel, your wall will look wavy. Fortunately the typical thicknesses are laid out for you on the page so you won’t have to look them up elsewhere.

Another key variable, which determines not just strength but also workability, is the mix ratio. For general render, a one-to-four cement-to-sand ratio is typical, though for a softer, more workable base coat, I’d tend towards a one-to-five, maybe even a one-to-six mix. Depending on your choice of ratio, the calculator divide the overall volume accordingly. Telling you exactly how much sand by cubic feet, and how many bags of cement you’ll require.

It accounts for dry volume allowance. This explains gap between the loose materials you heap into your wheelbarrow and actual volume of wet mortar. Because sand contains voids and cement absorbs water, you will need more dry material than the final wet volume. This is the quickest way to run short of materials mid-job, don’t forget it. You should of prepared more.

The final reality check is waste. No matter how careful an applicator, there will always be some mortar left on floor or in the hawk. To account for this, you can enter a percentage for waste into the tool; it should be higher for rough brick or uneven foundations than for smooth interior walls. Better to have some leftover than scrape last bag out from under your last full inch when your mortar begins to set up.

Because moisture content of sand varies widely (and so does the ambient humidity) we estimate water needs as a range, start at the low end and add sparingly until the mix sticks together but doesn’t slump.

Plaster is more than a math problem. It’s a sign that you respect how the stuff behaves. Once you have the numbers from the tool, then you know if the wall will stand up or not. Get the inputs correct to get there. The output is a solid map to a quality finish with minimum muss and fuss.