A progress indicator (PI) is often used to inform respondents of the task completion status of online surveys. When researchers conduct online surveys, reducing the dropout rate and the participants’ cognitive loads is important for improving the surveys’ efficiency. Although many studies have investigated the effect of using PIs with online surveys, it is unclear which PI design should be used to reduce the dropout rate and the participants’ perceived task load. Moreover, even in the Web content accessibility guidelines (WCAG), which should be followed in the building of websites to improve accessibility, the design guidelines for PIs are vague compared to those for other graphic elements. We noted that PIs have various designs and then created 25 types of PIs by combining design factors, labels, and bar graphs. We conducted an online survey through mTurk and collected 1,948 participants’ data. The online survey study results showed that the PI’s label design had a significant effect on the number of items the participants answered. In addition, we found that the respondents’ mental load changed significantly according to the combination of bar graph and label designs of the PI. This study provides strong support for designing the way a PI communicates the task process without reducing the speed of progress, which encourages respondents to answer more items and decreases their mental loads in online survey environments.