Does your elevation change how you make and can jam?

Yes. Above 1,000 feet, altitude changes two numbers in jam making, and they move in opposite directions. The temperature your jam must reach to set goes down, by about 2°F for every 1,000 feet [1]. The time your jars spend in the canner goes up, because water boils at a lower temperature and needs longer to do the same work [2]. Everything else in a tested recipe stays the same. This page shows both changes side by side, explains how to find your elevation, and works through the numbers for Boise, Eagle and the Treasure Valley foothills.

Both changes in one table

The set temperatures are USDA's [2]. The processing times are for half-pint and pint jars of jam and jelly in a boiling water canner, under the two systems in use: USDA's 5 minute method for jars sterilized first [2], and the 10 minute method for jars that weren't, with altitude added in bands [3][4].

Two rules come with the table:

1. Use the processing time and altitude steps from the same source as your recipe. Don't put one system's altitude additions on the other's base time. How long to process jam explains both systems and which recipes belong to each.
2. Set temperature only matters when you cook to temperature. Recipes with added pectin are timed (boil hard for the stated time once sugar is in), so altitude doesn't change their cooking. Jams and jellies made without added pectin are cooked until they reach the set, and that is where the altitude-adjusted temperature applies. Every recipe that is canned still needs the processing time for your elevation.

Why the boiling point drives both numbers

Water boils at 212°F at sea level and at a lower temperature as you climb, because there is less air pressure pushing down on it. That one fact explains both columns.

The set temperature falls. Jam sets when enough water has boiled off that the sugar concentration is high enough to gel. NCHFP measures that point relative to boiling water: a jam or jelly is done at 8°F above the boiling point of water [1]. At sea level that is 220°F. Higher up, water boils lower, so the target is lower too. A cook in Denver who waits for 220°F boils off too much water, and the jam sets stiff and gummy (Jam that's too stiff, gummy or sticky).

The processing time rises. By NCHFP's 2°F per 1,000 feet rule, a canner at full boil at 5,000 feet is about 10°F cooler than one at sea level [1]. Cooler water kills spoilage organisms more slowly and heats the jar contents more slowly, so the jars need more minutes to get the same treatment. Short jam processes add minutes in steps, by elevation band [2][4].

The same applies to sterilizing empty jars, when your recipe needs it: boil them 10 minutes below 1,000 feet, and add 1 minute for each additional 1,000 feet [3].

The thermometer trick that handles altitude for you

You don't need to know your elevation to find your set temperature. Bring a pot of water to a full boil, read your thermometer, and add 8°F [1]. If your thermometer reads 207°F in boiling water, your jam target is 215°F. This also corrects for a thermometer that reads a degree or two off and for the day's weather, which is why it is the better method. The temperature test gives the details.

The trick doesn't work for processing time. Canner times are set by elevation band, so you still need to know roughly how high you are.

Finding your elevation

Knowing which band you are in is enough; you don't need your elevation to the foot. Ways to find it:

• A phone altimeter app or an elevation website. SDSU Extension suggests a free altimeter app or whatismyelevation.com [4].
• Your city or county. Many cities publish an official elevation; your county extension office can tell you the range across the county.
• A topographic map. USGS maps show contour lines, which matter if you live on a hillside above town.

Watch for band edges. If your home sits within a couple of hundred feet of 1,000, 3,000 or 6,000 feet, check carefully, or use the higher band's time. Extra minutes in the canner don't make most jam unsafe, though they can soften a fruit low in pectin [5].

Worked example: Boise, Eagle and the foothills

The U.S. Geological Survey's field survey of the Boise area, made in 1895 and 1896, put the Boise River just below 2,500 feet at the western edge of its map and the valley floor near Table Rock at about 2,800 feet, with the foothills above rising to around 3,800 feet [6]. That is an old survey, so look up your own address before you rely on it: a phone altimeter app or an elevation website gives a figure in seconds [4]. Boise, Eagle, Meridian and Nampa sit on that same valley floor, and most valley homes fall in the 1,001 to 3,000 foot band.

A valley home at about 2,700 feet

A foothills home above 3,000 feet

The set temperature drops to about 214°F or a little lower. Processing with sterilized jars stays at 10 minutes, because USDA's band runs to 6,000 feet [2]. Processing with unsterilized jars rises to 20 minutes, the 3,001 to 6,000 foot band [4]. This is where foothill and valley cooks sharing a recipe can get different answers, and both are right.

Why unsterilized jars in the valley get 15 minutes

You may see 12 minutes quoted for Boise. It comes from a University of Georgia bulletin, which gives a 10 minute process for unsterilized jars and then says to add 1 minute per 1,000 feet [8]; at about 2,700 feet that works out to roughly 12 minutes. This page uses 15 minutes instead. NCHFP's own wording for unsterilized jars is a full 10 minutes "plus any altitude adjustments" [3], with no per-minute rule; the 1 minute per 1,000 feet rule NCHFP does publish is for sterilizing empty jars, a different job. Every published altitude adjustment for a filled-jar jam process steps by band: USDA adds 5 minutes at 1,000 feet [2], and the band chart in SDSU Extension's water bath publication adds 5 minutes from 1,001 to 3,000 feet [4]. Fifteen minutes agrees with both and is never shorter than what any source asks. If you want the shortest time for a soft-setting fruit such as strawberries or peaches, sterilize the jars and use USDA's 10 minutes instead [5].

What altitude doesn't change

• Ingredients. Fruit, sugar, acid and pectin stay exactly as written.
• Headspace. Still 1/4 inch for jam and jelly [2].
• Cooling. Jars still cool undisturbed for 12 to 24 hours [8].
• The rule against doubling. A bigger batch heats differently at any elevation.

Getting your numbers right at the stove

To get your set temperature from your elevation or your thermometer reading, use the Set temperature calculator. For processing minutes by elevation, jar method and recipe source, use the Processing time calculator. The full explanation of each number lives on The temperature test and How long to process jam.

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). https://nchfp.uga.edu/how/make-jam-jelly/jams-jellies-general-information/testing-jelly-without-added-pectin
2. USDA and National Center for Home Food Preservation, Complete Guide to Home Canning, Guide 7: Preparing and Canning Jams and Jellies, rev. July 2015. https://nchfp.uga.edu/papers/guide/GUIDE07_HomeCan_rev0715.pdf
3. National Center for Home Food Preservation, Do I need to pre-sterilize my jars before canning?, May 21, 2014. https://nchfp.uga.edu/blog/do-i-need-to-pre-sterilize-my-jars-before-canning
4. South Dakota State University Extension, Kleine and Botzet, A Guide to Water Bath Canning (P-00255), 2022. https://extension.sdstate.edu/sites/default/files/2022-10/P-00255-v2.pdf
5. UCCE Master Food Preservers of El Dorado County, University of California Agriculture and Natural Resources, Sterilizing Jars: When and Why, rev. July 2025. https://ucanr.edu/sites/default/files/2021-11/PIP-Jars%20sterilizing%20jars1-WhenWhy-revJuly2025.pdf
6. U.S. Geological Survey, Geologic Atlas of the United States, Boise Folio (Folio 45), Description of the Boise Quadrangle, from 1895 and 1896 field surveys. https://pubs.usgs.gov/gf/045/text.pdf
7. Penn State Extension, Zepp, How to Use an Atmospheric Steam Canner, updated March 17, 2025. https://extension.psu.edu/how-to-use-an-atmospheric-steam-canner
8. University of Georgia Extension, Andress, Preserving Food: Processing Jams and Jellies (FDNS-E-37-1), rev. June 2011. https://openscholar.uga.edu/record/23661/files/FDNS-E-37-1.pdf