Mansfield, Sean, K, Peiffer, Jeremiah, J, Hughes, Liam, J, and Scott Brendan, R. Estimating repetitions in reserve for resistance exercise: an analysis of factors which impact on prediction accuracy. J Strength Cond Res XX(X): 000–000, 2020—The purpose of this study was to examine the influence of knowing the load being lifted on the accuracy of repetitions-in-reserve (RIR) estimates, during both moderate- (60% 1 repetition maximum [RM]) and heavy-load (80% 1RM) exercise. Twenty trained men (age: 25.9 ± 4.5 years, height: 181 ± 7 cm, body mass: 86.5 ± 13.7 kg) were assessed for 1RM in bench press (98.4 ± 16.4 kg) and prone row (72.0 ± 11.7 kg), before being randomized into control (i.e., informed of the load; n = 10) or blinded (noninformed; n = 10) conditions. Subjects then completed 2 protocols in a randomized order: 3 sets at 80% 1RM and 3 sets at 60% 1RM. During each set of these protocols, subjects were asked to estimate their RIR before continuing the set to failure. Differences in estimated and actual RIR between sets and conditions were determined via 3-way repeated measures analysis of variance for the 60 and 80% 1RM protocols independently. No differences in RIR accuracy were observed between blinded vs nonblinded conditions. Repetitions-in-reserve estimates were lower than actual RIR for the first set of both exercises in 60 and 80% protocols (p ≤ 0.007, effect size [ES]: 1.30–2.89 [moderate-large]) and for set 2 of the 80% bench press protocol (p = 0.046, ES: 0.39 [small]). Knowing the load during resistance exercise or the %1RM of the load lifted did not influence the estimates of RIR. The ability to accurately determine RIR in the 60 and 80% 1RM protocols improved from sets 1–3, indicating that estimation of RIR is enhanced when an individual is estimating RIR at a closer point to actual failure.