Bois-à-Feu du Nord
Standardization of Firewood Values
We built a standardized system that derives firewood heat values (BTU), weights, and burn characteristics from universally measurable wood properties — density and specific gravity. This allows us to provide accurate firewood data for hundreds of species, far beyond the handful that have been tested in a laboratory.
Last updated: May 2026
The Problem: Limited Lab Data
Of the estimated 60,000+ tree species on Earth, only a few dozen have ever been tested in a laboratory for BTU output as firewood. Most published firewood charts repeat values for the same 20–30 popular North American species (oak, maple, hickory, etc.) and leave the vast majority of the world's woods unmeasured.
This means that if you burn a less common species — or you live outside North America — reliable heat-output data simply does not exist in published firewood guides.
We are the first and only platform to offer firewood values across such a huge variety of species, made possible by our standardized calculation approach.
Why Lab Testing Is Not Required for Firewood
The Wood Fuels Handbook (AIEL, Italy) established a key finding: the net calorific value (energy per kilogram) of oven-dry wood varies within a remarkably narrow range across all species:
- Hardwood (broadleaf): 18.5 MJ/kg
- Softwood (conifer): 19.0 MJ/kg
This means that a kilogram of oven-dry oak releases virtually the same energy as a kilogram of oven-dry birch, maple, or elm. The difference in heat per cord between species comes almost entirely from density — how much wood mass fits in a given volume.
Since density and specific gravity are routinely measured and published for thousands of species worldwide (for construction, furniture, and engineering purposes), we can derive accurate firewood heat values without needing an expensive lab calorimetry test for each species.
How accurate is this?
For the purpose of firewood — choosing between species, estimating how many cords to order for winter, or comparing suppliers — our calculations are more than accurate enough. Real-world variability (soil, climate, tree age, seasoning method) introduces far more variation than the narrow spread in calorific values between species.
Our Standardized Approach
We start with three reliable, widely-published properties for each wood species:
- Category — hardwood or softwood
- Specific gravity (SG) — the ratio of wood density to water density
- Dry weight — density in lb/ft³ and kg/m³ at 12% moisture content
From these three inputs, we derive every firewood value algorithmically: BTU per cord, seasoned weight, green weight, burn time, coal quality, ash production, and an overall rating.
Cord Volumes: How Much Solid Wood?
All our calculations are based on the actual solid wood content in a stacked cord, not the gross stacked volume:
Full Cord
- 128 ft³ total stacked volume (4 ft × 4 ft × 8 ft)
- 85 ft³ of actual solid wood (after removing air gaps, bark voids, and split irregularities)
Face Cord (⅓ cord)
- 42.67 ft³ stacked (4 ft × 8 ft × ~16 in)
- 28.33 ft³ of actual solid wood
We use the 85 ft³ solid-wood figure (not 128 ft³) in all BTU and weight calculations because wood-database.com reports density per solid cubic foot of wood — not per cubic foot of stacked firewood with air gaps.
How We Calculate BTU Values
Our BTU calculation follows the thermodynamic formula from the Wood Fuels Handbook, adjusted to 20% moisture content (the realistic target for well-seasoned firewood):
Step 1: Adjusted Energy per Kilogram
NCV at 20% MC = (NCV₀ × (1 − 0.20)) − (2.447 × 0.20)
This accounts for two effects of moisture: (a) water displaces burnable wood mass, and (b) energy is lost vaporizing the water during combustion. The 2.447 MJ/kg is the latent heat of water vaporization.
Hardwood: 14.31 MJ/kg at 20% MC · Softwood: 14.71 MJ/kg at 20% MC
Step 2: Convert to BTU
BTU/kg = NCV × 947.817 (1 MJ = 947.817 BTU)
BTU/lb = BTU/kg ÷ 2.20462
Step 3: Scale to a Full Cord
BTU/cord = seasoned weight per cord (lb) × BTU/lb
The seasoned weight per cord is derived from the published dry weight using our moisture normalization process (explained below).
Moisture Content: Why Our Numbers May Differ
This is the #1 reason our BTU values may look different from what you find on other websites. Most published BTU charts make one of these mistakes:
- Using oven-dry (0% MC) values — overstates heat by ~20% vs. what you actually get from seasoned firewood
- Using 12% MC without proper basis conversion — understates heat vs. reality
- Incorrectly converting between dry-basis and wet-basis moisture percentages
Our approach: Strict two-step normalization
Wood-database.com publishes weights at 12% moisture content on a dry basis (water mass ÷ oven-dry mass). Our firewood values target 20% MC on a wet basis (water mass ÷ total wet mass). These are fundamentally different measurement systems. Converting between them requires two steps — not one:
Step 1: Strip moisture to get oven-dry weight
Oven-dry weight = published weight ÷ 1.12
This removes the 12% water content from the published figure.
Step 2: Add moisture back at 20% wet-basis
Seasoned weight = oven-dry weight × 1.25
20% wet-basis equals 25% dry-basis (0.20 ÷ 0.80 = 0.25), so we multiply by 1.25.
Example: Red Oak at 44 lb/ft³ (12% MC)
44 ÷ 1.12 = 39.3 lb/ft³ oven-dry → 39.3 × 1.25 = 49.1 lb/ft³ at 20% MC Per cord: 39.3 × 85 = 3,341 lb oven-dry → 3,341 × 1.25 = 4,176 lb seasoned
⚠️ Skipping the oven-dry step (directly scaling from 12% to 20%) introduces an ~11% error that propagates into every BTU/cord figure.
How We Estimate Green (Fresh-Cut) Weight
The moisture content of freshly-cut wood varies significantly. Rather than guessing, we use a heuristic based on specific gravity — denser woods hold less water relative to their mass:
Hardwood Green Moisture (dry-basis %)
- SG ≥ 0.55 → 50% MC (dense hardwoods: oak, hickory)
- 0.35 ≤ SG < 0.55 → 70% MC (mid-density: cherry, birch)
- SG < 0.35 → 90% MC (light hardwoods: basswood, aspen)
Softwood Green Moisture (dry-basis %)
- SG ≥ 0.40 → 100% MC (dense softwoods: southern yellow pine)
- 0.25 ≤ SG < 0.40 → 150% MC (mid-density: spruce, fir)
- SG < 0.25 → 200% MC (very light: balsa, cedar)
Green weight per cord = oven-dry weight per cord × (1 + MC% ÷ 100)
These are estimates based on published research. Actual green MC varies by region, season, tree health, and how quickly after felling the wood is weighed.
Adjustable Moisture Content
Because our system keeps the oven-dry weight as the canonical reference point, we can recalculate the seasoned weight and BTU output at any target moisture content — not just 20%. This powers our adjustable MC feature:
Weight at target MC = oven-dry weight × (1 + MC_dry_equivalent) where MC_dry_equivalent = MC_wet ÷ (1 − MC_wet)
Want to see values at 25% MC? MC_dry = 0.25 ÷ 0.75 = 0.333 → multiply oven-dry by 1.333
This is why our system is uniquely flexible — change one number and every derived value (weight, BTU/cord, BTU/lb) updates correctly.
Why Our BTU Values May Look Different
If you compare our numbers to other firewood charts online, you may notice differences. Here's why:
We use 20% MC, not 0%
Many charts report oven-dry BTU values (0% moisture). Nobody burns oven-dry wood — it's unrealistic. Our 20% MC values reflect the heat you actually get from properly seasoned firewood.
We use correct basis conversion
Some sources incorrectly convert between 12% and 20% moisture without going through the oven-dry step. This introduces a systematic ~11% error.
We use proven conversion factors
Our constants (947.817 BTU/MJ, 2.447 MJ/kg latent heat, 85 ft³ solid wood per cord) come from peer-reviewed scientific references, not from approximations or rounded values.
We are consistent
Every species in our database is calculated using the exact same formula with the exact same constants. Many published charts mix lab-tested values, estimated values, and copied-from-other-charts values with inconsistent assumptions.
A First-of-Its-Kind Approach
Bois-à-Feu du Nord is the first and only platform to offer standardized firewood values for such a massive variety of wood species. Traditional firewood guides are limited to species that happened to be lab-tested decades ago. Our approach — grounded in the physics of combustion and the universality of wood energy content — means any species with published density data can receive accurate, comparable firewood ratings.
This is especially valuable for regions outside mainstream North American firewood territory — tropical hardwoods, exotic species, lesser-known local varieties — all receive the same rigorous, consistent treatment.
A Note on Real-World Variability
Our values are standardized estimates based on average published densities. Real-world firewood performance will vary because of:
- Climate and growing region (trees grow denser in cold climates)
- Soil fertility and moisture availability
- Tree age and growth rate
- Harvesting season (sap content varies through the year)
- Bark percentage and split dimensions
- Storage conditions and airflow during seasoning
- Standing dead vs. live-cut timber
- Fungal degradation or rot
- Stove or appliance efficiency (a 75% efficient stove captures 75% of the available BTU)
For practical firewood decisions — choosing species, estimating quantities, comparing suppliers — our standardized values provide a reliable baseline that is consistent and comparable across all species.
Sources & Scientific References
Every calculation on this page is grounded in peer-reviewed research and established reference data:
- Wood Fuels Handbook (AIEL — Italian Agroforestry Energy Association) — Baseline net calorific values (18.5 MJ/kg hardwood, 19.0 MJ/kg softwood) and the 2.447 MJ/kg latent-heat-of-vaporization coefficient.
- Wood Handbook (USDA Forest Products Laboratory, 2010) — Canonical reference for wood physical properties, including the dry-basis moisture content convention.
- Global Wood Density Database (Zanne et al., Dryad) — Comprehensive specific gravity reference with data for thousands of species worldwide.
- Fuelwood Characteristics of Some Important Tree Species — Cross-validation of calorific values across temperate hardwood and softwood species.
- The Wood Database (wood-database.com) — Per-species specific gravity and dried weight data, the primary input source for our derivation engine.
- How I Built My Firewood BTU Ratings (boisafeudunord.com) — The originating technical article documenting the complete derivation methodology.
Frequently Asked Questions
Are your BTU values less than what I see on other sites?
Often yes — because we report realistic 20% MC values. Most sites report oven-dry (0% MC) values which overstate the heat you'll actually get from seasoned firewood by about 20%.
Can I trust calculated values as much as lab-tested values?
For firewood purposes, absolutely. The variation between individual trees of the same species (due to soil, climate, age) is typically larger than the variation between our calculated value and a lab measurement. Lab testing matters for engineering applications — for heating your home, our standardized values are more than sufficient.
Why do you use 85 ft³ instead of 128 ft³ for a cord?
128 ft³ is the gross stacked volume of a cord. But a stacked cord contains air gaps, bark voids, and irregular splits. The actual solid wood is approximately 85 ft³ — and since density is measured per solid cubic foot, this is the correct multiplier for weight and BTU calculations.
What does "20% moisture content on a wet basis" mean?
It means that 20% of the total weight of the wood is water. This is different from "dry basis" where the percentage is relative to the dry-wood weight only. 20% wet-basis equals 25% dry-basis — this distinction is critical for accurate calculations.
