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 objects shaped similarly to protozoa demonstrated 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. Residuals (SP − E) were plotted against predicted (E) centered to the mean (X – mean) to evaluate for both mean and slope bias. For entodinia (ENTO), Y = 1.97 × 10⁴ (±1.48 × 10³) + 0.248(±0.0371)(X – 7.98 × 10⁴) μm³, with significant slope (P < 0.01) and mean (P < 0.01) bias. For isotrichids (ISO), Y = −1.21 × 10⁴ (±4.86 × 10⁴) − 0.124(±0.0685)(X – 2.54 × 10⁶) μm³, where slope trended toward significance (P = 0.08) with no mean (P > 0.10) bias. For epidinia (EPI), Y = 1.02 × 10⁵ (±1.46 × 10⁴) + 0.372(±0.219)(X – 1.45 × 10⁵) μm³, with no slope (P > 0.10) and significant mean (P < 0.01) bias. This demonstrates that SP more likely overestimates volume for ENTO or EPI than for teardrop-shaped ISO. This ellipsoid method offers potential to advance prediction of treatment effects on protozoal viability and volume.