Sap to syrup calculator: the Rule of 86 for maple sap

Every sugarmaker ends up asking the same two questions over a steaming pan: how much syrup is hiding in all this sap, and how long until it's done? This gauge answers both from three numbers. In the first box, enter the sap sugar content as a percent or in degrees Brix, which come to the same thing here. A sap hydrometer or a refractometer gives you the figure. Without one, sugar maple sap typically runs about 2%, which makes a fair first guess. The gauge takes anything from 0.5 to 6%.

Next, enter the gallons of sap you have collected. The last box, the evaporation rate in gallons of water per hour, is optional. Leave it empty and you still get the syrup and the water to boil off; fill it in and the gauge adds a boiling time. To find your own rate, keep the pan at a steady level for an hour and note how many gallons of sap you fed in to hold it there.

How it works

The gauge uses the Rule of 86, a sugaring rule of thumb that has been in use since the 1940s. Divide 86 by the sugar percentage and you get the gallons of sap needed for one gallon of finished syrup. Divide your sap by that ratio and you have the syrup. Everything else in the sap is water that has to leave as steam, so the water to boil off is the sap minus the syrup, and the boiling time is that water divided by your evaporation rate.

gallons of syrup = gallons of sap ÷ (86 ÷ sugar %)

The big figure in the readout, titled "Rule of 86 reading", is the ratio: gallons of sap per gallon of syrup. Under it, a banded scale from 0.5 to 4% shows where your sap sits. Below 1.5% is thin sap, with plenty of boiling for each quart. From 1.5 to 2.5% is typical sugar maple sap. Above 2.5% is sweet sap, with less boiling per gallon of syrup. Three lines follow: syrup from this sap, water to boil off, and boiling time.

Worked example

The gauge opens on 2% sap, 43 gallons of it, and an evaporator that drives off 10 gallons of water an hour. Since 86 ÷ 2 = 43, the readout shows 43 gallons of sap per gallon of syrup, and the marker sits in the band for typical sugar maple sap.

• Syrup from this sap: 1 gal (4 qt)
• Water to boil off: 42 gal
• Boiling time: about 4.2 h

Now change only the sugar. At 1.5%, the same 43 gallons gives 0.75 gal (3 qt) of syrup, and the ratio climbs past 57 to one. At 3%, it gives 1.5 gal (6 qt). Half a percent of sugar either way moves the yield by a quarter or more, which is why it pays to measure each batch rather than trust an old guess.

What changes the answer

The sugar in your sap. It varies from tree to tree and through the season, roughly 1 to 4% in sugar maples. Red, silver and black maples and boxelder can be tapped too, and their sap should be measured the same way. At 1% you need 86 gallons of sap for a gallon of syrup; at 4%, about 21.5.

Your evaporation rate. A shallow pan over a block arch and a wide evaporator boil at very different speeds, and the same rig slows down whenever the fire does. Dry, split wood keeps the boil hard, and our firewood cord calculator will tell you how much you have stacked for it. Treat the boiling time as an estimate for a steady boil, not a promise.

Where you call it done. The Rule of 86 assumes syrup finished to standard density. Syrup is done at about 7 °F above the boiling point of water where you are, which is about 219 °F at sea level, or at 66 to 67 °Brix on a syrup hydrometer. Water boils at a lower temperature as you climb and shifts a little with the weather, so check it on the day you boil rather than assuming 212 °F. Thin syrup stretches the yield and spoils sooner; overcooked syrup comes up short and can crystallize in the jar.

What you lose along the way. Sugar sand, the gritty mineral sediment also called niter, has to be filtered out, and some syrup always stays behind in the pan and the filter. Expect a home batch to come out a little under the figure on the gauge.

Boiling and tapping sensibly

Boil outdoors. In the default example, 42 gallons of water leave the pan as steam, and that much moisture has no business in a kitchen. Keep collected sap cold and boil it within a few days, since it sours quickly once the weather warms.

Sap runs on freeze and thaw days: nights below freezing and days in the 40s °F, often from late January into March depending on latitude and elevation. The season comes earlier in the South and later in the North, and our field almanac follows it through late winter. Common guidance is one tap for a tree about 10 to 12 inches across at chest height, with a second tap only on large trees. The same guidance favors 5/16 inch spiles in a hole about 1.5 to 2 inches deep, drilled slightly upward so the sap drains out. Mark your trees while the leaves are still on: red and sugar maples are among the trees that turn the ridges red in autumn, as our piece on fall color by elevation explains, and they are far easier to tell apart in October than in bare February.

Sources

• The Rule of 86, from C. H. Jones's 1946 work on maple sap and syrup, as used in university maple extension guides.
• Finishing points (about 7 °F above the local boiling point of water, or 66 to 67 °Brix) and tapping guidance from those same extension guides.
• Sugar content of maple sap: maple extension and research programs in the Northeast and Great Lakes states.

Questions

Is the Rule of 86 accurate enough for a backyard batch?

Yes. It is a rule of thumb, and small differences in finished density or losses in the filter will move your real yield a little. For planning how much sap to collect and how long to boil, it gets you close, which is why extension guides still teach it.

What if I don't know my sap's sugar content?

Start with 2%, the typical figure for sugar maple sap, and read the result as a rough guide. A sap hydrometer or refractometer gives you the real number in seconds. Measuring each batch also tells you which trees are the sweet ones worth tapping again.

Why did my batch come out short of what the gauge said?

The usual reasons are syrup boiled past standard density, syrup left in the pan and filters, and sap that was weaker than the number you entered. Check your sugar reading on the sap you actually boiled, not a sample from earlier in the run.

Can I boil sap on the kitchen stove?

A small finishing step indoors is common, but the bulk of the boiling belongs outside. Even 43 gallons of sap means 42 gallons of water going up as steam. Do the heavy evaporating outdoors and bring in only the nearly finished syrup to finish on the stove.