What temperature should my jam reach where I live?

Cook jam or jelly made without added pectin to 8°F above the temperature at which water boils on your own thermometer. At sea level water boils at 212°F, so the target is 220°F; for every 1,000 feet of elevation the target drops about 2°F, down to 205°F at 8,000 feet [1][2]. The most accurate way to use this calculator is to boil a pot of water, read your thermometer, and enter that number. If you would rather enter your elevation, it reads the USDA altitude table instead. Either way you get one target, in Fahrenheit and Celsius, and a check on whether your thermometer reads true.

Two ways in, and which one to trust

The boiling-water reading wins because the 8°F rule is the real rule and the table is a summary of it. A 65 percent sugar syrup boils a fixed amount above water, so what matters is where water boils in your pot that day [1][3]. The National Center for Home Food Preservation (NCHFP) tells you to check the thermometer in boiling water before you use it for exactly this reason [1].

The USDA altitude table

These are the end-point temperatures for jelly from USDA's Guide 7, the same values NCHFP publishes [2][1]. Guide 7 points jam made without pectin to the same table [2]. The Celsius figures are converted and rounded to one decimal.

Above 4,000 feet the table steps by a little less than 2°F per 1,000 feet (211°F at 5,000 rather than 210°F). The calculator follows the table as printed rather than the shorthand. Above 8,000 feet the table stops, so the calculator asks for your boiling-water reading.

A worked example at 2,700 feet

Say your kitchen sits at 2,700 feet. The table has rows for 2,000 feet (216°F) and 3,000 feet (214°F). Seven-tenths of the way between them is 214.6°F, which the calculator rounds to 215°F (101.7°C). It also tells you to expect water to boil near 206.6°F.

Now you boil water and your thermometer reads 210°F. The calculator switches to your reading: 210 + 8 = 218°F. It flags the gap. Your thermometer shows water boiling 3.4°F higher than the table expects, so it is probably reading high. If you had trusted the table and stopped at 215°F, the jam would have been about 3°F short of the set point and could stay runny. Cooking to 218°F on that thermometer reaches the real set point.

The same arithmetic works anywhere. If your thermometer shows water boiling at 207°F, the target is 215°F, whatever the elevation [1].

When the number does not apply

The temperature test is for jam and jelly cooked without added pectin, where you boil until the sugar is concentrated enough to gel [2]. Leave the thermometer in the drawer for these:

• Recipes with commercial pectin. These are timed. Guide 7's liquid and powdered pectin recipes bring the mixture to a full rolling boil and boil it hard for 1 minute [2]. The timed boil replaces the temperature test, so follow the recipe's minute. How long to boil each kind is on how long to boil jam.
• Low-sugar and no-sugar recipes made with low-methoxyl pectin, which sets with calcium rather than sugar and never reaches the high-sugar boiling point [4]. See low-sugar jam.
• Freezer jam, which is barely heated at all. See how to make freezer jam.

Getting an honest reading in the pot

The number on the screen is only as good as the reading in the pot. NCHFP asks for two things [1]:

1. Check the thermometer in boiling water before you start, which is the reading this calculator wants.
2. Keep the bulb fully covered by the boiling jam without letting it touch the bottom of the pan.

Small batches in wide pans are hard to read because the jam is too shallow to cover the bulb. Tilt the pan so the jam pools, or use a probe thermometer. Penn State notes that a 65 percent sugar solution boils at about the same temperature as finished jam, which is why the temperature test tracks soluble solids so well, and also why commercial makers confirm with a refractometer [3]. If you sell, the soluble solids check tells you whether a reading meets the 65 percent standard.

When the thermometer and the spoon disagree

Many makers run the temperature test and a second test together, because a thermometer can be off and a cold plate can mislead on a humid day. The sheet test, where two drops run together and slide off a cool metal spoon in a sheet, is set out step by step on the spoon (sheet) test, and which set test to use compares all four. The full explanation of the temperature method, including thermometer types, lives on the temperature test. Once the jam is in jars, the processing time calculator gives the canner time for the same elevation.

References

1. National Center for Home Food Preservation, Testing Jelly Without Added Pectin (adapted from So Easy to Preserve, 6th ed., 2014, Andress and Harrison), accessed October 2026. https://nchfp.uga.edu/how/make-jam-jelly/jams-jellies-general-information/testing-jelly-without-added-pectin
2. US Department of Agriculture and National Center for Home Food Preservation, Complete Guide to Home Canning, Guide 7: Preparing and Canning Jams and Jellies, revised July 2015. https://nchfp.uga.edu/papers/guide/GUIDE07_HomeCan_rev0715.pdf
3. Penn State Extension (Luke LaBorde), Food for Profit: Commercial Production of Jams, Jellies and Fruit Butters, updated May 24, 2024. https://extension.psu.edu/food-for-profit-commercial-production-of-jams-jellies-and-fruit-butters
4. University of Georgia Extension (Arnon, Henley and Schwan), Pectin for Home Food Preservation, Circular 1373, December 11, 2025. https://fieldreport.caes.uga.edu/publications/C1373/pectin-for-home-food-preservation/