We develop a method to calculate tsunami magnitude from ocean-bottom pressure gauge data around Japan. Direct application of Abe's equations to ocean-bottom pressure gauge data underestimates tsunami magnitudes by about an order of magnitude. This is because Abe's equations were derived only from tsunami amplitudes measured at coastal tide gauges where tsunamis are amplified by the shoaling of waves and reflection at the coastline. For offshore bottom-pressure records, we introduce correction factors to Abe's original equations to account for tsunami amplitudes measured in open oceans. The modified equations produce pressure gauge determined tsunami magnitudes close to those determined using coastal tide gauges.