kJ/mol to kcal/mol Converter
1 kJ/mol = 1 / 4.184 = 0.239006 kcal/mol. This reverse conversion appears when modern SI-based chemical data (kJ/mol) must be compared against legacy reference texts expressed in...
Formula
Source: IUPAC Green Book, NIST Chemistry Webbook | Last reviewed: June 27, 2026
Examples
1 kJ/mol
= 0.239 kcal/mol
1 kJ/mol = 0.239 kcal/mol
285.83 kJ/mol
= 68.32 kcal/mol
ΔH°f H₂O(l) converted to kcal/mol
436 kJ/mol
= 104.2 kcal/mol
H−H bond energy
30.5 kJ/mol
= 7.3 kcal/mol
ATP hydrolysis free energy
100 kJ/mol
= 23.9 kcal/mol
Moderate reaction enthalpy
Quick Reference Table
| kJ/mol | kcal/mol | Quantity |
|---|---|---|
| 4.184 | 1 | Reference: 1 kcal/mol |
| 100 | 23.9 | Moderate reaction enthalpy |
| 200 | 47.8 | Strong exothermic reaction |
| 285.83 | 68.32 | ΔH°f H₂O(l) |
| 436 | 104.2 | H−H bond dissociation energy |
| 890.4 | 212.8 | Methane combustion enthalpy |
Where is this used?
This reverse conversion appears when modern SI-based chemical data (kJ/mol) must be compared against legacy reference texts expressed in kcal/mol.
The kJ/mol is now the universal standard for thermochemical data in all major reference databases: the NIST Chemistry Webbook, the DIPPR 801 database, the DECHEMA chemistry data series, and the IUPAC critical data compilations all use kJ/mol as the primary unit.
However, the older literature, NBS Circular 500 (1952), the original JANAF Thermochemical Tables (1960s-1980s), and countless PhD theses and journal articles from the pre-SI era, used kcal/mol.
A process engineer designing a reactor and finding a reference that states the activation energy of a key side reaction as 'Ea = 30 kcal/mol' from a 1972 kinetics paper must convert the modern Computational Fluid Dynamics (CFD) simulation output (which reports the reaction rate in the Arrhenius form with Ea in kJ/mol) to verify the two data sources are consistent: 30 kcal/mol × 4.184 = 125.5 kJ/mol.
More specifically, when the CFD result gives Ea = 120 kJ/mol and the legacy paper gives 30 kcal/mol (125.5 kJ/mol), the 4.6% discrepancy must be investigated, is it due to different catalyst formulations, different temperature ranges, or genuine measurement uncertainty? In protein-ligand docking and drug design, binding free energies computed by molecular dynamics (MD) free energy perturbation (FEP) or MM-PBSA/MM-GBSA methods are reported in kJ/mol by default (Amber, GROMACS, NAMD, CHARMM output), but medicinal chemists think in kcal/mol: a binding affinity improvement of '2 kcal/mol' (8.4 kJ/mol) roughly corresponds to a 30× improvement in the IC₅₀ (inhibition constant at 298 K, using ΔG = −RT ln K).
The conversation between the computational chemist ('the FEP calculation shows ΔΔG = −11.3 kJ/mol for this methyl substitution') and the medicinal chemist ('so about 2.7 kcal/mol, that should give us ~100× better binding') requires fluent kJ/mol-to-kcal/mol conversion.
In electrochemical thermodynamics, the Nernst equation relates Gibbs free energy to cell potential: ΔG = −nFE, where F = 96,485 C/mol (Faraday constant).
A ΔG of −100 kJ/mol corresponds to a cell voltage of E = 100,000 / (n × 96,485) = 1.036/n volts.
Converting to kcal/mol: −100 kJ/mol = −23.9 kcal/mol, yielding the same E value because the Faraday constant unit tracks the energy unit.
The kJ-to-kcal factor is embedded in electrochemistry through the conversion of the gas constant R from 8.314 J/(mol·K) to 1.987 cal/(mol·K), and the relationship (RT/F) × ln(10) = 0.05916 V at 298 K (in base-10 log form of the Nernst equation) is the same whether you use kJ or kcal, because the conversion factor cancels in the ratio.
Real-World Usage Scenarios
Computational chemistry drug discovery project
A pharmaceutical company uses computational chemistry to optimize a lead compound for a new drug target. The computational team (using Schrödinger, MOE, or Open Babel) runs a free energy perturbation (FEP) calculation for a series of methyl group substitutions, reporting binding free energy differences (ΔΔG) in kJ/mol. The medicinal chemists, trained in the older literature, think in kcal/mol. The conversion: ΔΔG of −11.3 kJ/mol = −11.3 × 0.239 = −2.70 kcal/mol, approximately a 100× improvement in binding (using ΔG = −RT ln K, at 298 K: 1 kcal/mol ≈ 5.7× in K). The project team uses this predicted improvement to prioritize which methyl substitutions to actually synthesize and test. The kJ-to-kcal conversion (× 0.239) is the bridge between computational output and medicinal chemistry decision-making. A 1% error in the conversion (using 0.241 instead of 0.239) translates to about 1% error in the predicted ΔΔG, about 0.03 kcal/mol, or 17% error in the predicted binding improvement, meaningful for the synthesis prioritization.
Industry Standards Referenced
Frequently Asked Questions
Does the kJ/mol to kcal/mol conversion affect equilibrium constant calculations?
No, because the ratio ΔG/RT is dimensionless. If you use ΔG in kJ/mol, R = 0.008314 kJ/(mol·K). If you use ΔG in kcal/mol, R = 0.001987 kcal/(mol·K). The ratio is the same: ΔG/(RT) = (68.32 × 4.184)/(0.008314 × 298) = 68.32/(0.001987 × 298). The conversion factor cancels because both the numerator (kJ/kcal) and the gas constant R (in the denominator) scale proportionally. However, if you mix units, using ΔG in kcal/mol with R in kJ/(mol·K), the error is a factor of 4.184 in the exponent, which translates to exp(±4.184×ΔG/RT) error in K_eq. A ΔG of −10 kcal/mol at 298 K with mismatched R gives a K_eq error of exp(10 × 4.184 / 2.478 − 10 / 2.478) = exp(16.88 − 4.03) = exp(12.85) ≈ 380,000×, a catastrophic error. Always verify your units.
What does this converter do?
This converter performs the unit conversion at standard conditions using the exact conversion factor. The result is displayed with appropriate precision for engineering use.
How accurate is the conversion?
The conversion factor used is exact or to four significant figures depending on the units. For most engineering calculations, the precision is more than sufficient. For precision work, consult the relevant NIST or ISO standards referenced on this page.
Reviewed for accuracy
Verified against NIST standard reference data and IUPAC conventions · Last reviewed: June 27, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.