Previous observations of protozoa in cultures treated with essential oils or ionophores indicated possible cell shrinkage due to deleterious effects on cell function. Cell volume reduction by formaldehyde preservation combined with visually flattened or tapered morphology of rumen protozoa limited our ability to detect volume differences using common cylindrical derivations for protozoa. The advent of affordable, high definition imaging equipment enables recording of live protozoa from cultures treated with various additives that potentially shrink cells. We hypothesized that using still frames from video of protozoa swimming would improve accuracy of volume predictions by optimizing an approach to measure one maximal longitudinal measurement and both minimum and maximum diameter measurements perpendicular to the longitudinal axis, thus yielding a 3-dimensional estimation of protozoal volume. An ellipsoid formula (E, ⁴/₃πabc) was compared with previously published estimations using cylindrical (C, Lπ(^W/₂)²) or species coefficient (SP, XLW²) calculations. Testing this method on inanimate objects shaped similarly to protozoa demonstrated that the ellipsoid is more accurate in predicting volume as measured by displacement. True displacement was 11.8 mL for 10 large particles, and estimated volumes were 12.7 to 27.1, 7.7 to 16.6, and 12.3 mL for C, SP, and E, respectively. For smaller particles with more surface area, true displacement was 4.5 mL, and estimated volumes were 5.6 to 13.1, 3.2 to 7.5, and 5.6 for C, SP, and E, respectively. Rumen fluid sampled from 2 lactating Jersey cows was flocculated and wet-mounted on a microscope fitted with an HD (1080p) camera. Mean entodiniomorphid (ENTO) volumes were 109,665 (±55,912), 99,145 (±51,704), and 79,830 (±39,859) µm³ for C, SP and E, respectively. Mean epidinium (EPI) volumes were 1.74x10⁵, 2.47x10⁵, and 1.45x10⁵ for C, SP, and E, respectively. Regression of SP on E demonstrated that SP more likely overestimates volume for ENTO (Y = 1.187X + 0.8567) or EPI (Y = 1.467X + 38528) than for teardrop-shaped isotrichids (Y = 0.833X + 346,973). This ellipsoid method offers potential to advance prediction of treatment effects on protozoal viability and volume.