Peat deposits contain on the order of 1/6 of the Earth's terrestrial fixed carbon (C), but uncertainty in peat depth precludes precise estimates of peat C storage. To assess peat C in the Northern Highlands Lake District (NHLD), a approximately 7000 square km region in northern Wisconsin, United States, with 20 percent peatland by area, we sampled 21 peatlands. In each peatland, peat depth (including basal organic lake sediment, where present) was measured on a grid and interpolated to calculate mean depth. Our study addressed three questions: (1) How spatially variable is peat depth? (2) To what degree can mean peat depth be predicted from other field measurements (water chemistry, water table depth, vegetation cover, slope) and/or remotely sensed spatial data? (3) How much C is stored in NHLD peatlands? Site mean peat depth ranged from 0.1 to 5.1 m. Most of the peatlands had been formed by the in-filling of small lake basins (terrestrialization), and depths up to 15 m were observed. Mean peat depth for small peat basins could be best predicted from basin edge slope at the peatland/upland interface, either measured in the field or calculated from digital elevation (DEM) data (Adj. R2 = 0.70). Upscaling using the DEM-based regression gave a regional mean peat depth of 2.1 plus or minus 0.2 m (including approximately 0.1 to 0.4 m of organic lake sediment) and 144 plus or minus 21 Tg-C in total. As DEM data are widely available, this technique has the potential to improve C storage estimates in regions with peatlands formed primarily by terrestrialization. Number of sites: 21 Sampling Frequency: once for each site