Oil blend stability calculator
Blend up to eight carrier oils and see the combined fatty acid profile, the bis-allylic index that actually predicts rancidity, and a realistic shelf life band.
Choosing carrier oils is usually done by feel and by price. This calculator adds the third axis: how long the blend will survive before it smells of crayons. It works from published fatty acid ranges, and it ranks by the mechanism that actually drives rancidity rather than by total unsaturation.
Rancidity is driven by bis-allylic positions, the hydrogens sitting between two double bonds, not by unsaturation as such. Oleic acid has a double bond but no bis-allylic hydrogen, which is why oleic-rich oils behave almost like saturated fats on a balm's timescale. Score a blend as linoleic plus twice linolenic and lower is better.
How to use the number
| Bis-allylic index | Band | Indicative life | What to do |
|---|---|---|---|
| Under 10 | Very stable | 2 to 5 years | Nothing. This is as good as it gets |
| 10 to 25 | Stable | 18 to 24 months | Normal storage care is enough |
| 25 to 45 | Moderate | 12 to 18 months | Add mixed tocopherols, keep it dark and cool |
| 45 to 70 | Sensitive | 9 to 12 months | Reduce the fragile oil, or accept a shorter date |
| Over 70 | Fragile | 6 months or less | Use it as a small active fraction, refrigerate the raw oil |
Why not just use the iodine value?
Iodine value counts double bonds. It cannot see how they are arranged, and arrangement is what matters. A bis-allylic methylene, the carbon sitting between two double bonds in a 1,4-diene, loses its hydrogen easily because the resulting radical is delocalised over five carbons. The bond is roughly ten kilocalories per mole weaker than an ordinary allylic one. Oleic acid is unsaturated but has no such position, so it oxidises at a rate closer to a saturated fat than to linoleic.
Jojoba and meadowfoam are the cleanest demonstration. Both have plenty of double bonds and respectable iodine values, and both are extraordinarily stable, because in jojoba almost every chain is singly unsaturated and in meadowfoam the two double bonds sit at positions 5 and 13, too far apart to form a 1,4-diene. Measured induction periods bear this out: under identical accelerated conditions jojoba lasted about nine times as long as rosehip oil.
Published relative oxidation rates for oleic, linoleic and linolenic acid range from 1:12:25 to 1:70:265 depending on whether you measure oxygen uptake, hydroperoxide formation or propagation rate constants. The ranking is robust. The exact multiplier is not, which is why this tool gives a band rather than a date.
What matters more than the oil you chose
Packaging and temperature. In a controlled 270 day study, the same hemp seed oil in polypropylene at 25 C in diffused light passed three times the accepted peroxide limit by day 150, while in amber glass with foil at 10 C in the dark it stayed below the limit for the full nine months. That is a bigger effect than the difference between two moderately stable oils. Fill your containers full, keep them dark, keep them cool, and buy oil in sizes you will actually use.
The rest of the reasoning is on rancidity and oxidation, the full dataset is in the carrier oil comparison table, and shelf life testing covers how to set an honest date. If your balm already smells wrong, go to rancid balm. For the antioxidant question specifically, see vitamin E and antioxidants, and for how the blend will feel rather than how long it lasts, carrier oils compared.
Frequently asked questions
What is the bis-allylic index?
It is linoleic percent plus twice linolenic percent. A bis-allylic methylene is the hydrogen sitting between two double bonds, and it is the one that gets pulled off first in autoxidation. Counting those positions predicts rancidity far better than counting double bonds does.
Why does jojoba score so well when it has a high iodine value?
Because iodine value counts double bonds without seeing how they are arranged. Jojoba is a wax ester whose chains are almost all singly unsaturated, so it has essentially no bis-allylic positions at all. Measured induction periods put it far ahead of every triglyceride oil tested.
Are the shelf life bands measured?
No, and nobody's are. They are calibrated bands, not measurements, and the honest caveat is that packaging and storage temperature matter more than the oil you chose. The same hemp oil failed at 150 days in clear plastic at 25 C and stayed fresh for 270 days in amber glass at 10 C.
Will vitamin E fix an unstable blend?
It will slow oxidation that has not happened yet. It cannot reverse oxidation that has. And the useful dose is lower than most sources say: the measured optimum for alpha-tocopherol in oil is around 100 parts per million, above which it can turn pro-oxidant.
Do the percentages need to total 100?
Not for this tool. It normalises whatever you enter, so you can compare a three-oil blend without doing the arithmetic first. Your actual formula does need to total 100.
Reviewed and updated 6 September 2026. Spotted an error? Tell us and we will fix and log it.