Use the basis that the resin's data sheet gives for the mix ratio. If the sheet gives the ratio by weight, weigh both parts. If it gives the ratio by volume, measure both parts by volume.
The data sheet is the maker's technical sheet for the product. Part A is the resin and part B is the hardener. A resin mix by weight or volume is not a free choice. The two ratios are different numbers whenever part A and part B differ in density. The resin mix ratio calculator converts one into the other.
Why the two ratios are different numbers
Volume is the space a liquid fills, in milliliters (ml) or US fluid ounces (fl oz). Weight here means mass, in grams (g) or ounces (oz). Density links them. Density is the mass of one milliliter, in g/ml, and the data sheet lists it for each part.
In words: mass equals volume times density. With symbols: mass (g) = volume (ml) × density (g/ml).
If part A is denser than part B, one milliliter of A weighs more than one milliliter of B. A ratio by volume then needs a different set of numbers when you move to the scale. To go from volume to weight, multiply each volume part by that part's density. To go from weight to volume, divide each weight part by its density.
If both parts had the same density, the two ratios would match and the choice would not matter. You cannot know that from the numbers alone, so check the data sheet.
Worked example: converting a ratio both ways
The densities here are examples, not values for any real product. Part A is 1.10 g/ml and part B is 1.00 g/ml. The ratios 2 to 1 and 100 to 45 are also examples.
| Starting ratio | Part A | Part B | Result |
|---|---|---|---|
| 2 to 1 by volume, to weight | 2 × 1.10 = 2.20 | 1 × 1.00 = 1.00 | 2.20 to 1 by weight |
| 100 to 45 by weight, to volume | 100 ÷ 1.10 = 90.91 | 45 ÷ 1.00 = 45.00 | 90.91 ÷ 45 = 2.02 to 1 by volume |
Now a full batch. Take a ratio of 100 to 45 by weight and a total of 500 g. Part A is 500 × 100 ÷ 145 = 344.83 g. Part B is 500 × 45 ÷ 145 = 155.17 g. Dividing each by its density gives the volume of each part.
| Part | Grams | Ounces | Milliliters | US fl oz |
|---|---|---|---|---|
| A | 344.83 | 12.16 | 344.83 ÷ 1.10 = 313.48 | 10.60 |
| B | 155.17 | 5.47 | 155.17 ÷ 1.00 = 155.17 | 5.25 |
| Total | 500.00 | 17.64 | 468.65 | 15.85 |
One ounce is 28.3495 g and one US fluid ounce is 29.5735 ml. The mix ratio calculator gives these figures when you enter the ratio and both densities. For a larger pour, find the total first.
How to measure each way
By weight, on a scale
- Put the empty cup on the scale and tare it. To tare is to set the display to zero with the cup on it.
- Pour part A until the display shows its target mass.
- Tare again, then pour part B until the display shows its target mass.
- Stir for the time on the data sheet and scrape the sides and bottom of the cup.
A scale shows steps, such as 1 g. If the display moves in 1 g steps, the true mass can be up to 0.5 g off. On a 5 g target (an example), 0.5 g is 10%. Small batches need a finer scale, and the article on mixing small batches under 50 ml covers them.
By volume, in a marked cup
- Set the cup on a flat surface and read the line at eye level.
- Fill to the line for part A, then measure part B the same way.
- Stir for the time on the data sheet and scrape the sides and bottom of the cup.
You can check a cup's lines with water. One milliliter of water weighs about 1 g. Fill to the 100 ml line, weigh the water, and compare the reading with 100 g.
Mistakes and the size of each error
One slip is to read a weight ratio and measure it as volume, or the reverse. The error equals the ratio of the two densities. These rows use the example densities from above.
| Data sheet says | You do | What you actually mix | Size of the error |
|---|---|---|---|
| 100 to 45 by weight | Measure 100 ml and 45 ml | 110 g and 45 g, which is 2.44 to 1 instead of 2.22 to 1 | Part A is 10% too heavy (1.10 ÷ 1.00) |
| 2 to 1 by volume | Weigh 2 g and 1 g | 1.82 ml and 1.00 ml, which is 1.82 to 1 instead of 2 to 1 | Part A is 9.1% too small (1.82 ÷ 2) |
The article on why resin cures sticky looks at ratio errors like these in more detail.
Special cases
A volume ratio can be weighed once you convert it. Take a total of 300 ml at 2 to 1 by volume, with the example densities. Part A is 300 × 2 ÷ 3 = 200 ml, which weighs 200 × 1.10 = 220 g. Part B is 300 × 1 ÷ 3 = 100 ml, which weighs 100 × 1.00 = 100 g. The batch weighs 320 g, or 11.29 oz.
Changing the batch size keeps the ratio, not the amounts. Multiply both parts by the same factor, on the same basis. A batch twice as big needs twice the grams of A and twice the grams of B.
If you pour too much of part A on a scale, you cannot take it back, but you can raise part B to match. Take a target of 100 g of A and 45 g of B (an example). You pour 108 g of A. Part B must then be 108 × 45 ÷ 100 = 48.6 g, and the batch is 108 + 48.6 = 156.6 g. Check that your cup holds the larger total before you add part B.
Which basis to choose
| The data sheet gives | What to do |
|---|---|
| A ratio by weight | Weigh both parts on a scale. |
| A ratio by volume | Measure both parts by volume, or convert the ratio to weight with both densities. |
| Both ratios | Use either one, and use the same one for the whole batch. |
| A ratio but no density | Stay on the basis the sheet states, or find the densities as described below. |
Where the answer stops being reliable
A conversion is only as good as the densities you enter. Density changes slightly with temperature, so the figure applies at the temperature the data sheet states. The sheet may also round the ratio, for example to 2 to 1, and the calculator cannot see the unrounded value. The article on reading a resin data sheet shows where to find density, ratio and pour depth.
The sums also cannot tell you how much error the resin tolerates. That limit comes from the maker, and this article does not state it.
If the data sheet gives no density, you can find it yourself. Measure a known volume of one part, weigh it, and divide the mass by the volume. In an example where 100 ml of part A weighs 110 g, the density is 110 ÷ 100 = 1.10 g/ml. The reading is only as exact as the cup's lines, so check them with water first, and repeat for part B.