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

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)
  
• 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)

The total dry mass of the atmosphere is known quite closely, 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.
(2 ppmv CH₄ adds another 4.26 Gt carbon.)

(Click or hover over links for explanations and calculations.)

 

SeaLevel.info  

Last modified: 21-Jan-2026 (version 4)