Purpose: Develop a prediction equation for critical power (CP) and work above CP (W') in hypoxia for use in the work-balance (WBAL') model. Methods: Nine trained male cyclists completed cycling time trials (TT; 12, 7, and 3 min) to determine CP and W' at five altitudes (250, 1,250, 2,250, 3,250, and 4,250 m). Least squares regression was used to predict CP and W' at altitude. A high-intensity intermittent test (HIIT) was performed at 250 and 2,250 m. Actual and predicted CP and W' were used to compute W' during HIIT using differential (WBALdiff') and integral (WBALint') forms of the WBAL' model. Results: CP decreased at altitude (P < 0.001) as described by 3rd order polynomial function (R2 = 0.99). W' decreased at 4,250 m only (P < 0.001). A double-linear function characterized the effect of altitude on W' (R2 = 0.99). There was no significant effect of parameter input (actual vs. predicted CP and W') on modelled WBAL' at 2,250 m (P = 0.24). WBALdiff' returned higher values than WBALint' throughout HIIT (P < 0.001). During HIIT, WBALdiff' was not different to 0 kJ at completion, at 250 m (0.7 ± 2.0 kJ; P = 0.33) and 2,250 m (-1.3 ± 3.5 kJ; P = 0.30). However, WBALint' was lower than 0 kJ at 250 m (-0.9 ± 1.3 kJ; P = 0.058) and 2,250 m (-2.8 ± 2.8 kJ; P = 0.02). Conclusion: The altitude prediction equations for CP and W' developed in this study are suitable for use with the WBAL' model in acute hypoxia. This enables the application of WBAL' modelling to training prescription and competition analysis at altitude.