NASA Task Load Index
NASA-TLX measures perceived workload across six dimensions: Mental Demand, Physical Demand, Temporal Demand, Performance, Effort and Frustration. Rate each one straight after a task to get an overall workload score on a 0 to 100 scale. Add the optional pairwise weighting step to reflect which dimensions mattered most.
This is a free taste of the full ErgoSphere platform: the desktop app runs TLX sessions for whole teams, links the results to your task and risk registers, and rolls them into audit-ready reports.
Weight the Comparisons
The Raw TLX treats all six dimensions equally. For a Weighted TLX, compare the dimensions in pairs: for each of the 15 pairs, click the one that contributed more to your workload. A dimension's weight is the number of pairs it wins (0 to 5), and the six weights always sum to 15.
Workload Score
Complete all 15 pairwise comparisons to unlock the Weighted TLX. Until then the overall score uses the Raw TLX.
What is the NASA Task Load Index?
The NASA Task Load Index (NASA-TLX) is the most widely used subjective workload assessment tool in human factors. Developed by Sandra Hart and Lowell Staveland at NASA Ames Research Center in the 1980s, it measures the workload a person experiences by combining ratings across six dimensions, capturing not just how heavy the workload was but where it came from.
NASA-TLX measures perceived workload, not objective task difficulty. Two people doing the same task can report different workload depending on their experience, training and capacity, and that subjectivity is the point: the score records the demand actually felt by the operator. It is used across aviation, healthcare, defence, transport and industry.
Workload is not a single thing. A task can be physically easy but mentally heavy, or the reverse. By splitting workload into six dimensions, NASA-TLX captures the profile of demand, which tells you what to change, not just how much load there was.
When to use NASA-TLX
- Quantify the workload operators experience during a task or scenario
- Compare workload across interface designs, procedures or staffing levels
- Identify which dimension (mental, physical, temporal) is driving the load
- Provide evidence on operator loading for safety cases or regulatory submissions
When another approach may fit better
- You need a real-time, continuous measure during the task (consider ATWIT or physiological measures)
- The task is too brief for a meaningful impression to form (under about 30 seconds)
- Participants cannot reliably reflect on their own experience
The Six Dimensions and the Score
Each dimension is rated from 0 to 100 immediately after the task. Note that Performance is reversed: its left end is Good and its right end is Poor, so a low number means good perceived performance.
| Dimension | What it captures | Endpoints |
|---|---|---|
| Mental Demand | Thinking, deciding, calculating, remembering, looking, searching | Low / High |
| Physical Demand | Pushing, pulling, turning, controlling, activating | Low / High |
| Temporal Demand | Time pressure from the pace at which task elements occurred | Low / High |
| Performance | How successful you felt in accomplishing the task goals | Good / Poor |
| Effort | How hard you had to work, mentally and physically, to perform | Low / High |
| Frustration | How insecure, discouraged, irritated, stressed or annoyed you were | Low / High |
Raw TLX and Weighted TLX
The Raw TLX is simply the average of the six ratings, treating every dimension equally. The original Weighted TLX adds a step: you compare the dimensions in all 15 possible pairs and pick the one that contributed more each time. A dimension's weight is the number of pairs it wins (0 to 5), the six weights always sum to 15, and the overall score becomes the weighted average. Hart (2006) showed Raw TLX correlates highly with the weighted version, so either is defensible.
Reading the result
NASA-TLX has no official thresholds. These practitioner bands are a common guide and match the colour coding used by this calculator:
| Score | Workload level | Interpretation |
|---|---|---|
| 0 - 24 | Low | Demands well within capacity. No intervention required. |
| 25 - 49 | Moderate | Noticeable but manageable. Monitor for sustained operations. |
| 50 - 74 | High | May degrade performance over time. Consider redesign. |
| 75 - 100 | Very high | At or beyond capacity. Immediate intervention recommended. |
Always read the score alongside the dimension profile. A score of 65 driven by mental demand needs different fixes (decision aids, simpler displays) than the same score driven by temporal demand (better pacing, extra staffing at peak).
Frequently Asked Questions
What does NASA-TLX measure?
NASA-TLX measures perceived workload, that is how much demand a person felt while doing a task. It combines ratings across six dimensions: Mental Demand, Physical Demand, Temporal Demand, Performance, Effort and Frustration, to give an overall workload score on a 0 to 100 scale plus a profile of where the demand came from.
What is the difference between Raw TLX and Weighted TLX?
Raw TLX is the simple average of the six dimension ratings, treating each one equally. Weighted TLX first runs 15 pairwise comparisons so the dimensions that mattered most to the task carry more weight. Research shows the two correlate highly, so Raw TLX is widely used when time is short.
What is a good or bad NASA-TLX score?
NASA-TLX has no official cut-off values. As a practical guide many practitioners read 0 to 24 as low, 25 to 49 as moderate, 50 to 74 as high and 75 to 100 as very high workload. Always interpret a score against the specific task and operator, and compare conditions rather than relying on the absolute number.
Why is the Performance scale reversed?
On the Performance dimension the left end is Good and the right end is Poor, so a low number means good perceived performance. This reversal is part of the original NASA-TLX design and should be explained to participants before they rate.
Is this NASA-TLX calculator free?
Yes. It runs in your browser with no sign-up, gives an instant Raw TLX score and an optional Weighted TLX, and exports a result image. It is a free taste of the full ErgoSphere platform, which runs TLX sessions for whole teams and links results to your task and risk registers.
Further Reading
- Hart, S. G., & Staveland, L. E. (1988). Development of NASA-TLX (Task Load Index): Results of empirical and theoretical research. In P. A. Hancock & N. Meshkati (Eds.), Human Mental Workload (pp. 139-183). North-Holland. doi.org/10.1016/S0166-4115(08)62386-9
- Hart, S. G. (2006). NASA-Task Load Index (NASA-TLX); 20 years later. Proceedings of the Human Factors and Ergonomics Society Annual Meeting, 50(9), 904-908. doi.org/10.1177/154193120605000909
- Bustamante, E. A., & Spain, R. D. (2008). Measurement invariance of the NASA TLX. Proceedings of the Human Factors and Ergonomics Society Annual Meeting, 52(19), 1522-1526. doi.org/10.1518/001872008X288394
- Grier, R. A. (2015). How high is high? A meta-analysis of NASA-TLX global workload scores. Proceedings of the Human Factors and Ergonomics Society Annual Meeting, 59(1), 1727-1731. doi.org/10.1016/j.apergo.2015.03.010
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