Specific surface area can be a strong predictor of organic carbon (SOC) contents in soils. Specific surface area can be estimated reliably and cost‐effectively from water adsorption by air‐dry soil samples, but SOC itself can also adsorb water. For estimating the mineral component of specific surface area, it is, therefore, necessary to exclude water‐adsorption by SOC. Here, we refer to “apparent specific surface area” for measurements that include water adsorption by both mineral soil and SOC. We used a mathematical approach to estimate water adsorption by SOC so that this component can be subtracted from measurements of apparent specific surface area. We used a dataset of apparent specific surface area and soil carbon at seven depths from 50 soil cores collected from a research farm in the Manawatu region in New Zealand. Both apparent specific surface area and SOC content decreased with soil depth with very high correlation (r2 = 0.98). We estimated the SOC contribution to apparent specific surface area from the slope of the relationship between changes in apparent specific surface area and SOC content. For our soils, the SOC contribution to apparent specific surface area was estimated as 0.43 ± 0.02 m2 mgC−1. This parameter allows apparent specific surface area measurements to be corrected for the water adsorption by SOC to calculate the functionally relevant mineral specific surface area. Highlights Soil surface area can be estimated from the H2O content of air‐dry soil but SOC also adsorbs H2O. We developed a mathematical approach to estimate water adsorption by SOC. We estimated the contribution of SOC to apparent specific surface area as 0.43 ± 0.02 m2 mgC−1. Mineral specific surface area can be inferred by subtracting SOC‐based H2O adsorption.