How Much Resin Do I Need for My Mold?
Two ways this goes wrong, and I’ve done both. You mix too little, the pour comes up short, and by the time you’ve mixed more the first batch has started to skin over — now you’ve got a line in your piece. Or you mix way too much, fill the mold, and watch the leftover go off in the cup as expensive trash. Getting the amount close is its own small skill.
Here’s how I figure out how much to mix, including the cheap trick that takes the guessing out of it.
The water trick (the one I actually use)
This is the most reliable method and it costs nothing. Fill your empty mold with water right up to where you want the resin to sit. Pour that water into your mixing cup and read the volume on the cup’s markings. That number is how much total mixed resin you need — water and resin take up nearly the same space.
Then dry the mold completely (water and resin do not get along — a damp mold hazes the pour). Mix your resin and hardener to that total volume, in the correct ratio, and you’ll land almost exactly right.
I do this for any new mold or any piece where I care about the result. It turned “mix some and hope” into a number, and my leftover waste dropped to almost nothing.
Reading the cup: total, not each part
One thing that trips beginners: the volume you measured is the total mixed amount, A and B combined. So if your mold needs 30 ml total and your resin is 1:1 by volume, that’s 15 ml of resin plus 15 ml of hardener. For a 2:1 resin, the same 30 ml is 20 ml resin plus 10 ml hardener. Get the ratio right within the total — the ratio rules matter.
A small habit that helps: mix a hair more than the water told you — maybe 5–10% extra. Resin clings to the cup and stir stick, so a little is always lost on the way to the mold. Better a tiny bit left than a short pour.
When you can’t do the water test
Sometimes you’re estimating before the mold’s in front of you, or it’s an open-backed piece. A rough mental model helps.
Resin is sold and measured by volume or weight, and roughly 1 fluid ounce ≈ 30 ml, and most casting resins weigh close to 1.1 grams per ml, so 30 ml is around 33 g. For flat work you can also estimate from area and depth — a coaster about 4 inches across (10 cm) poured 1/4 inch (6 mm) deep needs somewhere near 60–70 ml. These are ballparks, not gospel; the water test beats them every time it’s available.
Deep or chunky molds are where estimating fails hardest, and they’re also where over-mixing hurts most — a big batch of resin generates its own heat as it cures, and too much at once can overheat and yellow or crack. For those, pour in thinner layers rather than one giant mix.
A few habits that save resin
Keep a little notebook (or a note on your phone) of how much each mold actually took — after one pour you never have to guess that mold again. I have a running list for my regulars, and it’s saved me more wasted resin than any other single thing.
And mix in a cup that’s bigger than you need, not snug to the brim. Crammed-full cups are hard to stir and scrape properly, and incomplete mixing gives soft spots no matter how perfect your measurement was. Room to move your stick is room to get a clean cure.
Browse my handmade silicone molds — many list their capacity.
Got a specific mold you’re trying to fill? Tell me its rough size and depth and I’ll give you a starting amount to mix.
— Nikolai