This research demonstrates (1) an Internet-based forest carbon measurement system to support Tier 3 level carbon stock assessments; and (2) a method that links ground-based measurements or IPCC Tier 1 biomass values with Earth Observation satellite remote sensing to map carbon across large areas at Landsat-class (15-metre) spatial resolution. We demonstrated web-based tools and carbon mapping methods in two case study areas: Sangthong District, Lao PDR and Bac Kan Province, Viet Nam. Results for carbon stock in the forest strata for these two pilot areas are reported via an online system using geographic parcels and plot information, and tree inventory data collected at the plot level. The carbon mapping method utilises a vegetation continuous fields (VCF) algorithm, which is used to downscale IPCC Tier 1 default biomass values or stratified Tier 3 biomass values to compute carbon at 15-metre Landsat pixel resolution. The Internet-based forest carbon system and mapping method is a globally-deployable, scalable MRV (measurement, reporting and verification) system that is cost-effective and uses rigorous scientifically-valid methods for calculating carbon. The system is online at http://mrv.carbon2markets.org.