Convert Kilojoules to Calories - Energy Converter
Convert energy values from kilojoules (kJ) to dietary Calories (kilocalories, kcal) using SI-consistent unit definitions. This tool applies the standard thermochemical relations used in metrology and nutrition science.
For clarity: in nutrition contexts a Capital 'Calorie' (Cal) = 1 kilocalorie (kcal). The converter returns kilocalories unless you specifically need small calories (cal).
Governance
Record 7f52ed9a9276 • Reviewed by Fidamen Standards Committee
Interactive Converter
Convert between kilojoule and kilocalorie with precision rounding.
Quick reference table
| Kilojoule | Kilocalorie |
|---|---|
| 1 kJ | 4.18 kcal |
| 5 kJ | 20.92 kcal |
| 10 kJ | 41.84 kcal |
| 25 kJ | 104.6 kcal |
| 50 kJ | 209.2 kcal |
| 100 kJ | 418.4 kcal |
Methodology
The conversion is based on the thermochemical definition that 1 calorie (small cal) = 4.184 joules (J). By extension, 1 kilocalorie (1 kcal or 1 Cal) = 4,184 J and 1 kilojoule = 1,000 J. We apply these fixed SI relationships without empirical estimation.
This converter uses the precise factor consistent with NIST metrology: 1 kJ = 1000 J, and 1 kcal = 4184 J, so kcal = kJ / 4.184. Results are presented with sensible rounding for readability and scientific traceability.
Worked examples
100 kJ → 23.9005736 kcal (≈ 23.90 kcal)
4184 kJ → 1000 kcal (by definition: 4184 ÷ 4.184 = 1000)
1 kJ → 0.239005736 kcal (≈ 0.2390 kcal)
F.A.Q.
Are you converting to small calories (cal) or dietary Calories (kcal)?
This converter returns dietary Calories (kcal). In nutrition contexts 'Calorie' (capital C) equals 1 kilocalorie (kcal). If you require small calories (cal), multiply kcal by 1000 or use the small-calorie conversion provided above.
What is the exact numeric factor used?
We use the thermochemical definition: 1 kcal = 4184 J and 1 kJ = 1000 J, so 1 kJ = 0.239005736 kcal (kcal = kJ / 4.184). This follows NIST conventions for unit definitions.
How should I round results for nutrition labels or recipes?
For diet and food labels, follow the regulatory rounding rules of your jurisdiction (for example, FDA and other national agencies provide guidance). For informal use, two decimal places for kcal is usually sufficient; for scientific reporting, keep more significant figures and state uncertainty.
Does the tool account for measurement uncertainty from calorimetry?
No. This tool performs exact unit conversion using fixed constants. Measurement uncertainty from experimental calorimetry (instrument calibration, heat losses, sample variability) must be handled upstream and reported separately according to metrology best practices (for example, NIST guidelines).
Can I rely on this for regulatory nutrition labeling?
Use this converter for accurate unit conversion only. For regulatory nutrition labeling, follow your local authority's methodology for energy calculation and rounding (for example, FDA in the United States or comparable national agencies). Cross-check with official nutrition databases when calculating food energy.
Why do some sources quote 4.186 J per calorie instead of 4.184 J?
Different definitions (thermochemical vs. International Table) historically produced slightly different values. The internationally adopted thermochemical value used by modern metrology and nutrition references is 4.184 J per small calorie. Use NIST and official standards for authoritative values.
How do I convert back from kcal to kJ?
Reverse the formula: kJ = kcal × 4.184. This is the exact SI-consistent relationship used in scientific and regulatory contexts.
Sources & citations
- NIST Reference on Units and Constants (SI Unit relationships) — https://physics.nist.gov/cuu/Units/current.html
- U.S. Food and Drug Administration — Food Labeling & Nutrition — https://www.fda.gov/food/food-labeling-nutrition
- USDA FoodData Central (energy values and nutrient database) — https://fdc.nal.usda.gov
- U.S. Department of Energy — Energy Basics and Units — https://www.energy.gov/science-innovation/energy-sources
- MIT OpenCourseWare — introductory physics resources (energy unit context) — https://ocw.mit.edu
- ISO 80000-4:2019 — Mechanics — https://www.iso.org/standard/64975.html
- NIST SP 811 — Guide for the Use of the International System of Units — https://www.nist.gov/pml/special-publication-811
Further resources
Related tools
Versioning & Change Control
Audit record (versions, QA runs, reviewer sign-off, and evidence).
Record ID: 7f52ed9a9276What changed (latest)
v1.0.0 • 2025-11-10 • MINOR
Initial publication and governance baseline.
Why: Published with reviewed formulas, unit definitions, and UX controls.
Public QA status
PASS — golden 25 + edge 120
Last run: 2026-01-23 • Run: golden-edge-2026-01-23
Versioning & Change Control
Audit record (versions, QA runs, reviewer sign-off, and evidence).
What changed (latest)
v1.0.0 • 2025-11-10 • MINOR
Initial publication and governance baseline.
Why: Published with reviewed formulas, unit definitions, and UX controls.
Public QA status
PASS — golden 25 + edge 120
Last run: 2026-01-23 • Run: golden-edge-2026-01-23
Engine
v1.0.0
Data
Baseline (no external datasets)
Content
v1.0.0
UI
v1.0.0
Governance
Last updated: Nov 10, 2025
Reviewed by: Fidamen Standards Committee (Review board)
Credentials: Internal QA
Risk level: low
Reviewer profile (entity)
Fidamen Standards Committee
Review board
Internal QA
Entity ID: https://fidamen.com/reviewers/fidamen-standards-committee#person
Semantic versioning
- MAJOR: Calculation outputs can change for the same inputs (formula, rounding policy, assumptions).
- MINOR: New features or fields that do not change existing outputs for the same inputs.
- PATCH: Bug fixes, copy edits, or accessibility changes that do not change intended outputs except for previously incorrect cases.
Review protocol
- Verify formulas and unit definitions against primary standards or datasets.
- Run golden-case regression suite and edge-case suite.
- Record reviewer sign-off with credentials and scope.
- Document assumptions, limitations, and jurisdiction applicability.
Assumptions & limitations
- Uses exact unit definitions from the Fidamen conversion library.
- Internal calculations use double precision; display rounding follows the unit's configured decimal places.
- Not a substitute for calibrated instruments in regulated contexts.
- Jurisdiction-specific rules may require official guidance.
Change log
v1.0.0 • 2025-11-10 • MINOR
Initial publication and governance baseline.
Why: Published with reviewed formulas, unit definitions, and UX controls.
Areas: engine, content, ui • Reviewer: Fidamen Standards Committee • Entry ID: 55634c1e3ded