Specific Gravity – Advanced Candle Math
The craft of candle making intersects with scientific concepts pretty frequently.
Chemistry describes how candle materials come together to support combustion on top of the wick and math helps us measure the right amount of materials for our jars.
Thermodynamics helps explain how wax behaves when it changes state from a solid to a liquid.
Candle makers have built systems and formulas over the years on these concepts to perfect their craft.
Perfect candles built from experience and a deep understanding of how every material weaves together to form a melody of wick and fire.
Although we’ve simplified the formula for measuring materials, the logic has a pretty significant flaw if left undiscovered.
Every wax-fragrance-dye combination is different, and each batch of wax varies in its characteristics.
The math formula’s accuracy depends on the precision of a single variable.
Most candle makers substitute an average number. If you take time to lock this variable down, your measurements will become more accurate than ever before.
That variable is the Specific Gravity of your wax blend.
What Is Specific Gravity?
Specific gravity is a fancy way to describe density, and gives us a good way to figure out how much something weighs when it fills a certain space.
Makes a lot of sense since we measure out wax by weight in an attempt to fill a volume.
Weight and volume are tied together by density – if you know two of them, you can always find the third.
Density is our behind-the-scenes actor making sure the formula we use to measure wax is accurate and precise.
More on that in a second.
What is specific gravity and how does it relate to density?
Don’t let the words fool you.
Even though they sound complicated, they aren’t. Specific gravity describes the density of something compared to water.
It’s just a relative number.
Water always has a specific gravity of 1.0.
If you find something with a specific gravity of 2.0, that means it’s twice as dense as water (it would fill half the space as water but weigh the same).
Remember, objects and supplies that are really dense seem to be really heavy without taking up much space.
Candle wax specific gravity ranges from 0.80 up to 0.98, depending on the wax blend included in the formula. A higher specific gravity means more wax is required to fill a space.
Fragrance oils have an even larger range, which explains why some bottles from suppliers contain less than others: they sell by weight, not volume.
Oils with a higher specific gravity are more dense and fill less space than a fragrance with more top notes and a lower specific gravity.
A fragrance oil with a high specific gravity will fill less of a bottle than a lower specific gravity but weigh the exact same.
So how can you use this concept for candle making?
The Original Formula
If you know your fragrance load (f, %) and your Total Weight (TW, grams/ounces) you can calculate everything with two math formulas:
Wax (W)
W = TW ÷ (1 + f)
Fragrance Oil (FO)
FO = W · f
…which all seems straightforward and easy except that finding the Total Weight is prone to errors. With nothing else to go on, Total Weight can be estimated by filling a container (or mold) with water then multiplying by 0.86.
What is 0.86? That’s an average specific gravity for candle supplies. You can find it by dividing 3 by 3.5, too (which is commonly how it’s suggested).
It’s very unlikely to accurately represent your true materials, which is where this guidance comes in.
Finding the specific gravity of your materials allows you to tune the formula for your exact use case.
No more estimates or assumptions.
You can lock down the required measurements with a higher level of precision if you know the specific gravity of what you’re measuring, and it’s easier than you might think.
How To Find Specific Gravity
Put on your best science apron – it’s time to hit the laboratory for a candle experiment!
To find a specific gravity for a material you need to know how much something weighs (weight) and how much space it fills (volume).
Fortunately, most people own a measuring glass (for volume), and almost every candle maker has a scale (for weight).
You won’t get very far without either of these.
Step 1 - Prepare Your Equipment
You’ll need something to measure volume, and something to measure weight.
Weight measurements simply require a scale. If you make candles, you should already have this.
Volume measurements require a container with mL, cups listed on it.
A measuring glass works, but a lab beaker with mL works even better because they tend to be more precise.
Step 2 - Melt Wax
Eventually, we’re going to weigh the liquid in a container (step 3).
If you’re measuring for fragrance oil, you can skip directly to Step 3 because it’s already a liquid.
Solid wax is often shipped in flakes or blocks, which makes measuring volume trickier because of air and compaction. Blocks are closer, since you can measure volume by multiplying width, height, and depth, but only if the block has a constant width, height, and depth.
If it’s less of a rectangular shape and more “artistic” looking (AKA, non constant measurements), you should probably melt it down.
Melt enough wax to fill the volume-measuring-device (beaker or measuring glass) to a specified level.
A little extra is okay as long as you make sure you can fill it to a clear measurement in the glass.
Don’t add fragrance oil or dye.
Even though your final wax blend is technically composed of fragrance oil, dye, AND wax, you’ll likely use different sets of ingredients in the same wax.
Also, your final blend is mostly wax anyways, since fragrance loads don’t usually exceed 10%-12% of the candle, even in heavy-load applications.
Note that you don’t have to heat the wax to any specified temperature, just melt it enough so there is no solid left and you can transfer it into another container without worrying about it solidifying too quickly.
Step 3 - Fill Measuring Device To A Specific Volume
This step is easy.
Before transferring any wax, make sure to tare the weight on your scale for the measuring glass. If you can’t tare weight, then record the measuring glasses empty weight to subtract later.
After you’ve completely melted down the wax, pour it into the container with measurements on it until it matches a listed “tick”, like 500 mL or 2 cups and weigh it on the scale.
You can fill it to any actual fill height, as long as you’re able to see exactly how much material is in it.
The larger, the better.
You should have two numbers at this point – weight and volume.
Congratulations!
We can find the specific gravity/density of the material.
Make sure you track the units of measuring each:
- Weight: ounces or grams
- Volume: mL or cups
You can use other units, but they aren’t as common on most measuring devices.
Step 4 - Convert To Specific Gravity
If you have the right tools, this step isn’t as tricky as it appears. First, find your density by dividing weight by volume.
Density (D)
Density = Weight ÷ Volume
Once you have Density, divide it by the number in the “Divide By” column below to find the Specific Gravity:
| Unit of Weight | Unit of Volume | Divide By |
| grams | mL | 0.997 |
| grams | cups | 235.88 |
| ounces | mL | 0.03517 |
| ounces | cups | 8.32 |
For example, if you measured wax or fragrance oil weighing 429 grams filled up to the 500 mL mark:
Density = 429 grams ÷ 500 mL = 0.858 grams/mL
Based on the chart above, you would divide density by 0.997 to find the Specific Gravity:
Specific Gravity = 0.858 grams/mL ÷ 0.997 = 0.86
This math works for wax, fragrance oil, and fragrance oil blended with wax (even though that’s not usually recommended).
An Updated Formula
All this math helps you determine the Total Weight that fits into a container or mold.
How does Specific Gravity help with that?
Specific Gravity tells you how to transform water weight into wax weight.
If you know the weight of water in a container or mold, you can find the equivalent wax weight to fill the same space using it.
Since water is easy to find and doesn’t make a mess, determining the Total Weight for any container is as simple as measuring the water weight and multiplying by the specific gravity – that’s all!
- Fill container or mold with water and measure the weight. Make sure to tare the container or subtract out the container weight if you can’t tare your scale.
- Find the specific gravity of your wax blend using the steps outlined above.
- Multiply water weight by the specific gravity of your wax to find TW (Total Weight)
- Substitute Total Weight (TW) into the equation below to finalize the measurements for your wax and fragrance.
Wax (W)
W = TW ÷ (1 + f)
Fragrance Oil (FO)
FO = W · f
Remember, f is always a decimal in the above equation so if you choose to use an 8% fragrance load, f = 0.08 (or 8 ÷ 100, if that’s easier to remember).
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