Purpose: The aims of this study were to 1) compare active drag (Da) calculation between a single land-based measurement of frontal surface area (FSA) and in-water FSA measures obtained at key events of the arm pull (1, right upper-limb catch; 2, right upper-limb insweep; 3, right upper-limb exit and left upper-limb catch; 4, left upper-limb insweep; and 5, left upper-limb exit and right upper-limb catch) at front crawl swimming, and 2) compare mechanical power variables computed based on these two approaches. Methods: Seventeen swimmers (11, male; 6, female; 16.15 ± 0.94 yr old) were recruited. The FSA was measured based on two approaches: (i) nonvariation, that is, assuming a constant value, and (ii) variation, that is, calculated in each key event of the front crawl swim. Active drag based on a nonvariation of the FSA was measured using the Velocity Perturbation method. Active drag based on a variation approach was measured in each key event of the front crawl according to the law of linear motion. Paired t-test (P ≤ 0.05), simple linear regression models, and Bland–Altman plots between assessment methods (variation vs nonvariation) were computed. Results: The FSA (variation) was higher than when assuming a nonvariation (0.1110 ± 0.010 vs 0.0968 ± 0.010 m2, Δ = 15.69%, t = 4.40, P < 0.001, d = 0.95). Active drag (variation) was also significantly higher than when assuming a nonvariation (88.44 ± 25.92 vs 75.41 ± 15.11 N, Δ = 16.09%, t = 3.66, P = 0.002, d = 0.61). Conclusions: Besides the FSA, swim velocity also changes during the front crawl arm pull. The variation of both variables had a significant effect on the active drag measurement and consequently on mechanical power and total power input variables.