“PgC” or “GtC” (carbon) vs. “Gt CO₂” vs. “ppmv CO₂”
conversion factors

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tl;dr:

1 ppmv CO₂ = 7.8024 Gt CO₂, which is 2.1294 PgC (≡ GtC)
to convert from PgC to Gt CO₂ multiply by 3.6642

Details:

Per https://ciaaw.org/atomic-weights.htm (2024), in normal materials:

• average atomic weight of carbon = [12.0096, 12.0116] = 12.0106 ±0.0010 g/mol
  
• average atomic weight of oxygen = [15.99903, 15.99977] = 15.99940 ±0.00037
  
• so, the average molecular weight of CO₂ = [44.00766, 44.01114] = 44.00940 ±0.00174  (or 44.0095 according to NIST)
  
• which is [3.663930, 3.664497] times the atomic weight of carbon
• so, to convert from PgC to Gt CO₂ multiply by [3.663930, 3.664497] = 3.6642 ±0.0003
  (Note: the GCP  rounds this to 3.664)
• that also means CO₂ is [0.272889, 0.272931] = 27.291 ±0.002 percent carbon

The total dry mass of the atmosphere is known quite closely (5.1352 ±0.0003 × 10¹⁸ kg), from which we can calculate:
1 µmol/mol CO₂ in dry air, a/k/a “1 ppmv CO₂” or “1 ppm CO₂” ≅ 7.8024 ±0.0005 Gt CO₂ or 2.1294 ±0.0003 PgC

Example:

427 ppmv CO₂ masses 427 ppmv × (7.8024 ±0.0005 Gt/ppmv) = 3331.625 ±0.214 Gt.
It contains 427 × (2.1294 ±0.0003 GtC/ppmv) = 909.25 ±0.13 Gt carbon.

(Likewise, 2 ppmv CH₄ masses 5.69 Gt, of which 4.26 Gt is carbon; see here.)

(Click or hover over links for explanations and calculations.)
(For other conversion factors see:  https://sealevel.info/conversion_factors.html )

 

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Last modified: 24-Aug-2026 (version 10)