Density and hardness: how closely they track
By Lucas Anderson · Published · Data: Wood Handbook 2010
Heavier woods are almost always harder. Across the 155 woods in the Wood Handbook tables, specific gravity and Janka hardness have a correlation of 0.95, the closest link between any two measures on this site. The exceptions are still large enough to matter: two woods of identical density can differ by 57% in hardness.
How close the link is
Plot specific gravity against Janka hardness for all 155 woods and the points form a tight band rising from left to right. The correlation coefficient is 0.95, where 1 would be a perfect straight line. Within each group it stays high: 0.95 for U.S. hardwoods, 0.92 for U.S. softwoods and 0.93 for imports. For comparison, density correlates 0.93 with bending strength, 0.85 with stiffness and only 0.43 with volumetric shrinkage. Of the measures on this site, hardness is the one density predicts best.
A rule of thumb from the data
A straight line fitted through the points gives Janka hardness ≈ 4,708 × specific gravity − 1,300 lbf. Read it as a rough guide, not a formula to quote. At a specific gravity of 0.50 it predicts about 1,050 lbf; black walnut, at 0.51, measures 1,010 lbf. Each 0.10 of specific gravity adds roughly 470 lbf. The line accounts for about 90% of the variation in hardness across the list (the square of the correlation). What it leaves out is still sizeable: the typical distance between a wood’s real hardness and the line, measured as the standard deviation of those gaps, is about 230 lbf.
The same pattern shows in bands of density. The median hardness climbs steadily from one band to the next, while the range inside each band stays wide:
| Specific gravity | Woods | Median lbf | Range, lbf |
|---|---|---|---|
| 0.20–0.29 | 2 | 280 | 240–320 |
| 0.30–0.39 | 33 | 430 | 350–630 |
| 0.40–0.49 | 44 | 755 | 430–1,080 |
| 0.50–0.59 | 31 | 1,210 | 660–1,510 |
| 0.60–0.69 | 26 | 1,665 | 1,230–2,140 |
| 0.70–0.79 | 8 | 2,325 | 1,710–3,170 |
| 0.80–0.99 | 11 | 3,150 | 2,040–3,680 |
Harder than their weight suggests
The interesting woods are the ones furthest from the line. These six measure the most above the hardness the rule of thumb predicts for their density:
| Wood | SG | Janka | Predicted |
|---|---|---|---|
| Indian rosewood | 0.75 | 3,170 | 2,230 |
| Marishballi | 0.88 | 3,570 | 2,840 |
| Manbarklak | 0.87 | 3,480 | 2,800 |
| Ipe | 0.92 | 3,680 | 3,030 |
| Bubinga | 0.71 | 2,690 | 2,040 |
| Azobe | 0.87 | 3,350 | 2,800 |
Indian rosewood is the clearest case: at a specific gravity of 0.75 the line predicts about 2,230 lbf, and the handbook lists 3,170 lbf. All six are among the ten hardest woods in the tables, so the largest gaps above the line sit at the hard, heavy end of the scale.
Softer than their weight suggests
| Wood | SG | Janka | Predicted |
|---|---|---|---|
| Keruing | 0.69 | 1,270 | 1,950 |
| Caribbean pine | 0.68 | 1,240 | 1,900 |
| Karri | 0.82 | 2,040 | 2,560 |
| Goncalo alves | 0.84 | 2,160 | 2,650 |
| Determa | 0.52 | 660 | 1,150 |
| Kapur | 0.64 | 1,230 | 1,710 |
Keruing and Caribbean pine sit furthest below the line. Both are fairly heavy, at 0.69 and 0.68, but measure only 1,270 lbf and 1,240 lbf. Pairs with identical density show the same thing: bubinga and kempas both have a specific gravity of 0.71, yet bubinga measures 2,690 lbf and kempas 1,710 lbf, a 57% difference.
Where the line breaks down
A straight line also fails at the light end. For ceiba, the lightest wood in the tables at a specific gravity of 0.25, it predicts −120 lbf, which is impossible; the real figure is 240 lbf. So the rule of thumb is most useful in the middle of the range, roughly between specific gravities of 0.35 and 0.75, where 125 of the 155 woods sit.
Inside a single family the link can be looser or tighter than overall. Across the 15 oaks the correlation between density and hardness is 0.93; across the 13 pines it is 0.99. Small groups give less stable figures, so treat these as a description of this data, not a law.
Which density figure
Density on this site is specific gravity: the oven-dry weight of the wood divided by the weight of water that would fill its volume. For U.S. species the handbook bases it on green volume. It also lists a second value based on volume at 12% moisture content for 89 U.S. woods, and that figure is higher by a median of 8.6%, because dry wood has shrunk. For the imported woods the handbook does not state which basis it used, which adds some uncertainty when an import is compared with a U.S. wood of similar density.
When density is enough
If all you know about a wood is its weight, density will put it in the right part of the hardness scale most of the time. For a choice between two woods of similar density it is not enough, because a few hundred pounds-force either way is common. The handbook publishes hardness for all 155 woods here, so there is no need to estimate: the comparer shows both figures together, and the finder filters by a density range and a Janka range at once. More on the hardness figure itself is in how to read a Janka number.
Sources and method
USDA Forest Service, Forest Products Laboratory. Wood Handbook: Wood as an Engineering Material (2010, FPL-GTR-190), Tables 4-3 and 4-4 (shrinkage) and 5-3b and 5-5b (strength and hardness). Public domain. Medians, counts, ranks, correlations and percentages are calculated by TimberLedger from those tables across the 155 woods on this site. Data as of: Wood Handbook 2010 edition (published test averages of small clear specimens). How we use the data.