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Cotton yield monitor accuracy can be defined in numerous ways. Field accuracy or field error is the accuracy over an entire field and is most commonly used by sales people because it is usually the smallest number. This is due to measurement errors often averaging themselves out over an entire field. Load accuracy or load error is the accuracy over a picker basket load of cotton. Instantaneous accuracy is the accuracy of each yield data point and is very difficult to measure. To better quantify instantaneous accuracy, we bagged and weighed cotton passing by three cotton yield monitor systems at 3, 5, and 7 second intervals. Bagged weights were compared to yield monitor data recorded during the same intervals. Plots were set up for continuous, or steady-state, conditions as well as step-input conditions. An additional, modified step-input test was conducted which had 3 segments of cotton removed among 4 segments of cotton. Overall, we found that instantaneous accuracy during steady-state conditions was not affected by yield/weight. Harvest interval length (plot length) did have some effect on instantaneous accuracy with the longer interval (7 s) having lower mean errors for all the systems. Accuracy errors ranged from 0.7% to 26.3%. The mean absolute yield error for each system was 9.6% (Ag Leader), 11.0% (AGRIplan), and 11.5% (MCYM). Both the Ag Leader and AGRIplan systems appear not to record data for some interval after harvest begins, making it difficult to evaluate response under step-input conditions.