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I'm currently mixing render to cover ugly brick walls in my front garden.

Let's say I want to use this recipe for the top coat, just for an example:

  • Cement 1 parts
  • Lime 1 parts
  • Sharp Sand 2 parts
  • Soft Sand 2 parts

The exact recipe isn't critical but my question is ... well a 25 kg bag of lime is much bigger in volume than a 25 kg bag of cement or sand, about twice or close enough, so are the recipes I'm finding online by weight or by volume? None of them say.

Now when I'm mixing mortar I just use shovels (Volume), and that seems to be the advice I'm finding online and cement and sand are pretty close in weight/volume so it's fine, but now I'm seeing this huge difference in the weight/volume of lime I am concerned that the recipes will be way off if I don't break this question once and for all.

Also, I've read all the articles that say it doesn't matter because sand and cement have similar density, eg: Understanding render mix ratios but please remember that this won't cut it when lime is involved because the densities are definitely different.

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  • 2
    Think the word "parts" means more volume than weight. If weight most people would tend to use weight measures(lbs, kgs), especially if the densities were much different. Good question, hope an expert chimes in.
    – crip659
    Jul 6 at 0:52
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    @knowitall which weighs more: a pound of sand or a pound of feathers? Now how about a cubic foot of sand vs a cubic foot of feathers? So can you really say that the ratio is the same when comparing volume to weight?
    – brhans
    Jul 6 at 1:31
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    My hunch is weight, because when it comes to these kinds of quantities, you typically don't measure it so much as "add 'x' bags of each item". And the bags (at least what I have seen) of these types of materials are measured by weight, not volume. But that's just a hunch, because until I saw this post, I thought render had to do with chicken fat. Jul 6 at 1:51
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    @knowitall the problem you're skipping over is that the common "recipes" do not give measurements in pounds or shovels or buckets. They use "parts". So if you're measuring these "parts" in volume and you mix 2 gallons of sand with 1 gallon of lime you get a different ratio of sand to lime compared to measuring "parts" by weight and using 2 pounds of sand with 1 pound of lime - because sand and lime have different densities.
    – brhans
    Jul 6 at 1:54
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    The density of cement is 1.44 g/cm³, while the density of hydrated lime is 2.21 g/cm³. If I mix 2 parts cement with 1 part lime by weight - lets say 20g cement + 10g lime and then look at their respective volume I have 13.9cm³ of cement and only 4.5cm³ of lime. So even though I mixed 2:1 by weight I now have roughly 3:1 by volume.
    – brhans
    Jul 6 at 2:52

2 Answers 2

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Disclaimer: I am NOT an expert on cement, mortar, render or other brick laying techniques/technology.


I think the confusion comes from the fact that you're looking to mix in small quantities.

If you were mixing a large quantity, you'd mix by the bag:

  • 1 bag of cement
  • 1 bag of lime
  • 2 bags of sharp sand
  • 2 bags of soft sand

and you wouldn't think twice about, nor would you care, that the bag of lime was much bigger (other than grumbling as you tried to heft it into the mixer).

If you need to make a "half recipe" because you don't need that much, you'd use 1/2 bag of cement & lime and 1 bag of each of the sands.

Since you seem to be interested in making even smaller quantities, you now need to measure, somewhat accurately, your materials. Since you're simply working in "parts", and you now want to measure small quantities, you need to know how to measure the parts.

I think it stands to reason that you'd continue to measure in fractional bags. Since the bags are labeled by the manufacturer by weight, not volume, it would stand to reason that you want to continue to measure your material by weight and not make the switch to volume.

So, a small recipe would now be:

  • 1/25 bag (or 1 Kg) of cement
  • 1/25 bag (or 1 Kg) of lime
  • 2/25 bag (or 2 Kg) of sharp sand
  • 2/25 bag (or 2 Kg) of soft sand

There's no switching of units of measure because your original "bag" measures were all 25Kg units.

NOTE: If the bags are labeled by volume not by weight, then the exact opposite is true: You'd need to make your partial recipe by volume, but it would still be a "fractional bag".


Disclaimer: Again, I am NOT an expert on this matter, I've just worked through it in a way that seems logical to me.

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  • ...but if mixing large quantities, you don't have bags of sand, normally. You have a pile deposited by a dump truck. And a mixer. And a shovel...
    – Ecnerwal
    Jul 6 at 13:11
  • @Ecnerwal and as a result you'd probably mix it wrong at least '(technically' anyway). But since the relative densities of the common materials aren't that much different, and it's probably rare to use materials with a very different density, the resulting mix when done by volume is usually close enough. If you're making a mortar mix with something like lime, then you almost certainly don't have a pile of lime lying on the ground, but rather have it in bags portioned by weight.
    – brhans
    Jul 6 at 13:15
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    valid point, @Ecnerwal, but I've also seen plenty of videos of guys pulling bags of material from large stacks and dropping them into the mixers, too. And, if you've got dump truck loads of material, you may also have a bucket loader instead of a shovel. (At least, I'd hope so!)
    – FreeMan
    Jul 6 at 13:20
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    Well done. It's interesting to note that ready-mix concrete is sold by volume (cubic yards in USA) but is batched by weight at the plant (have a look at a delivery ticket!). One could (and I have) done the same with a bathroom scale and bucket. That said, for small jobs it's common practice to use an informal "lots of this, some of that, mix with water until it looks right" recipe. The reality is that concrete works with a wide range of ratios. Often it's not necessary to optimize for cost, nor to guarantee the correct 28-day strength, and so "good enough" is ... good enough.
    – Greg Hill
    Jul 6 at 16:03
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    Good points, @GregHill. If you're pouring the foundation for a 52 story sky scraper then you'd better get the mix right. If you're making a skim coat for an existing brick wall as a decorative overlay, it's probably not quite as critical.
    – FreeMan
    Jul 6 at 16:08
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It's nearly always volume with building materials. The reason this is true is mostly one of practicality. It's much more of a hassle to weigh raw materials on a jobsite than to measure or estimate volume. While recipes depend on density to some extent, they're set up to be applied by volume.

Some examples...

  • Masonry is mixed by shovelful, bag, or cubic yard (sand, gravel, cement). While bags are filled by weight, the quantity of bags in a recipe is more about volume.
  • Tile mortar is mixed by volume parts (dry powder and water), though some products imply both (quarts of water to a bag of powder weighing N lbs.). Really you're dealing with volume in either case.
  • Paint is colored and thinned by liquid volume, irrespective of density (base, colorants, thinners).

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