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Firewood species and seasoning: what extension services and Forest Research say

This page sets out the species heat tables from the University of Maryland and South Dakota, the drying study Maryland ran on four hardwoods, and the calorific values Forest Research publishes for logs, chips and pellets.

Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.

This page reports what the named codes, regulators and makers say. It is not training and does not teach the work. This page reports what the official source says. It is not legal advice: ask the office named here. The moisture line and the 2 cubic metre rule mentioned below are England's, set out on the England firewood sale page. The courses that teach tree work are on the certifications guide. Cord and log volumes can be worked out with the firewood stack tool and the log weight tool.

Heat value by species

University of Maryland Extension (fact sheet FS-937, 2012) says heat value for firewood is expressed in British Thermal Units, and that species differ. It adds that hardwoods such as oak are denser and harder to split but sustain longer fires and give more heat, and that softwoods ignite quickly. South Dakota's Department of Agriculture forest health update of December 12, 2018 (by the state forest health specialist, who is also the extension forester) gives a ranking in million BTU per cord of seasoned wood. The two tables use different species names and do not agree, so they are set side by side and not merged. The Maryland sheet credits its table partly to a chart from a stove retailer.

SourceSpeciesMillion BTU per cord
Maryland FS-937Oak25.3
Maryland FS-937Ash23.6
Maryland FS-937Walnut21.8
Maryland FS-937Elm21.4
Maryland FS-937Sycamore20.7
Maryland FS-937White pine19.0
South Dakota updateBur oak, mulberry25
South Dakota updateHoneylocust, sugar maple24
South Dakota updateBlack walnut22
South Dakota updateCrabapple21
South Dakota updateGreen ash, hackberry20
South Dakota updateAmerican elm19
South Dakota updateBoxelder17
South Dakota updatePonderosa pine15
South Dakota updateAspen, cottonwood14
South Dakota updateBasswood13

The South Dakota update says oak gives almost twice the heat of basswood or cottonwood, that cottonwood goes to ash quickly, that oaks and sugar maple make excellent coals and that pines throw a lot of sparks. It defines a cord as a stack 4 feet wide, 4 feet high and 8 feet long holding 128 cubic feet of space and 70 to 80 cubic feet of solid wood. Maryland gives the same 128 cubic feet and puts the solid wood between 65 and 90 cubic feet, with wetter wood taking up 8 percent more space than dry wood. Both sheets add that a pick-up load is a fraction of a cord that varies with the truck: Maryland says 1/5 to 1/2 of a cord and South Dakota says 1/5 to 1/3.

Drying time in the Maryland study

University of Maryland Extension fact sheet FS-1074 (November 2018) reports a study on four hardwoods in the Piedmont and Blue Ridge of Western Maryland: black locust, hickory, black cherry and hackberry. The trees were alive when felled in mid-April 2016, cut to rounds 16 to 18 inches long, split, stacked on pallets in the shade until mid-morning and left uncovered. Moisture was measured monthly for a year.

  • By November, 6.5 months after harvest, all four species stood between 20 and 26 percent. The sheet says black locust and black cherry were ready to burn then, at sustained averages of 20 to 21 percent.
  • Hackberry and hickory dried more slowly, at 23 to 26 percent after 6.5 months. After a year hickory still averaged 24 percent, which the sheet puts down to its density.
  • The sheet says the other species were acceptable at 7 months, within the 6 to 9 months it says is suggested for drying firewood. It states that oak was not in the study and would likely dry like hickory or black locust.
  • In a University of Maryland research study the sheet mentions, unsplit hickory lengths left uncovered still held over 30 percent moisture after one year, while split pieces reached 20 percent after 6 months of drying.

FS-1074 gives the conventional view as wood split, stacked, covered and left for 6 to 12 months, or until the inside reads 20 percent, and says climate, harvest season and species all change that. FS-937 says seasoning starts at splitting, not at felling, and that green split wood needs 6 to 9 months stacked and covered. The EPA Burn Wise page, a frozen record since October 30, 2025, says wood should be seasoned for at least six months and burned at under 20 percent moisture. England's Ready to Burn rule uses 20 percent or less as its legal line for small volumes.

Calorific values from Forest Research

Forest Research publishes a table of typical net calorific values. It gives log wood, stacked and air dry at 20 percent moisture, 14.7 GJ per tonne (4.1 kWh per kg), a bulk density of 350 to 500 kg per cubic metre and 1,400 to 2,000 kWh per cubic metre. For wood chips at 30 percent moisture it gives 12.5 GJ per tonne and 250 kg per cubic metre, for oven-dry solid wood 19 GJ per tonne, and for wood pellets at 10 percent moisture 17 GJ per tonne. The table is by moisture and form, not by species.

Sources