Where the rule of 86 comes from
Divide 86 by the sugar content of the sap and you get the gallons of sap it takes to make one gallon of syrup. Two percent sap gives 43 to one. That is the whole rule, and it holds because the sugar in the tank has to end up in the jug — nothing is created or destroyed on the way, only water leaves.
The constant itself is not magic. It falls out of the sugar content syrup is finished to and the fact that finished syrup is a good deal denser than sap: a gallon of syrup weighs around eleven pounds against roughly eight and a third for sap. Run those two figures together and you land in the mid eighties. Different sources use 86 or 87 depending on the finishing figure they assume, which is why the constant is a field on this page rather than a number baked into it. What syrup has to test at where you are is set by your state, and this page does not know it.
Half a point of sugar is worth more than a bigger pan
This is the part that surprises people. The volume of water you have to boil off is almost exactly the volume of sap you collected, because the syrup you get back is a rounding error against it. So the boiling time barely changes when the sap gets thinner — but the syrup you get out of that same boil drops hard.
| Sap sugar | Sap per gal of syrup | Water off, per gal of syrup | Hours at 8 gal/hr |
|---|---|---|---|
| 1.0% | 86.0 gal | 85.0 gal | 10.6 |
| 1.5% | 57.3 gal | 56.3 gal | 7.0 |
| 2.0% | 43.0 gal | 42.0 gal | 5.3 |
| 2.5% | 34.4 gal | 33.4 gal | 4.2 |
| 3.0% | 28.7 gal | 27.7 gal | 3.5 |
Going from 2.0 to 1.5 percent adds about a third to the hours behind every gallon of syrup. Going the other way, from 2.0 to 2.5, cuts them by a fifth. No change to the arch, the pan or the wood pile does anything like that. Which trees you tap, and how the season is running, is the lever — and it is largely not under your control, which is exactly why it is worth measuring rather than assuming.
Concentrating moves the water, it does not make syrup
Reverse osmosis pulls water out cold, ahead of the pan, and leaves the sugar behind. Take two percent sap up to eight and you have quartered the volume going to the fire while the syrup at the end is identical. The calculator shows both halves so the saving is visible as hours rather than as a vague claim: sugar in equals sugar out, and the membrane only decides where the water leaves.
What it does not do is change the yield. If a page or a salesman implies more syrup from the same sap, the arithmetic says otherwise.
Fuel, honestly
Published evaporation rates are for a pan running hard with a well-managed fire, and a first-season operation rarely hits them. Time your own run: mark a level in the tank, boil for an hour with the fire the way you normally keep it, and measure the drop. Do the same for fuel with a weighed tank or a stacked and measured woodpile. Numbers from a catalogue will make the plan look better than the season does.
If you are burning wood, the firewood cord calculator converts a stack into cords and the firewood seasoning and BTU calculator covers what moisture content does to the heat you actually get — wet wood in a sugarhouse is a slow boil and a long night. For how many taps produce the sap in the first place, the maple tap count calculator works forward from the trees.
Questions people ask
Is it 86 or 87?
Both are used, and the difference between them is under two percent on the answer. The constant comes from the sugar content syrup is finished to and the density difference between sap and syrup, so a source that assumes a slightly different finishing figure quotes a slightly different constant. It is a field on this page for that reason. What matters far more is measuring the sap sugar accurately: an error of a tenth of a point on a two percent reading moves the answer five percent, which is four times the gap between 86 and 87.
Why does thinner sap barely change my boiling time?
Because you are boiling water, and almost all of the sap is water either way. Forty three gallons of two percent sap has forty two gallons of water in it; fifty seven gallons of 1.5 percent sap has fifty six. Per gallon of sap the boil is the same. What changes is how much syrup sits at the bottom of it when you are done, and that is what falls. So thin sap does not cost you a longer night, it costs you the syrup you expected at the end of a normal one.
How many taps do I need for a gallon of syrup?
That depends on how much sap a tap runs in your woods and season, which varies enormously — tree by tree, year by year, and with whether you are on buckets or on tubing under vacuum. Work it from your own records rather than a general figure. The calculation is straightforward once you have a sap-per-tap number you trust: total sap divided by sap per tap. Your state maple producers association and the extension service both publish regional figures if you have no history to work from.
Does a bigger evaporator make more syrup?
No. It makes the same syrup sooner. Yield is fixed by the sugar you collected, and the pan only decides how many hours the water takes to leave. A bigger rig matters when sap is arriving faster than you can boil it and you are running out of storage, which is a real constraint in a warm week — but it is a throughput problem, not a yield one. Adding sugar content, or concentrating before the pan, is what shortens the hours behind each gallon.
Can I sell what I make?
Selling changes the question from a hobby one to a regulatory one, and this page cannot answer it. Density and grading standards, labelling, licensing and the food-safety rules that apply to a sugarhouse are all set at state level and they differ. Contact your state department of agriculture and your local health department before you sell rather than after, because retrofitting a building or a process to meet a rule is far more expensive than building to it.