Effort can be measured subjectively (how hard something feels), physiologically (what your body is doing), or both. The best method depends on whether you’re tracking physical exertion during exercise, mental workload during a task, or overall strain over time. Each approach captures a different dimension of effort, and combining two or more gives you a far more accurate picture than relying on any single metric.
The Perceived Exertion Scale
The simplest way to measure physical effort is to rate how hard you feel you’re working. The Borg Rating of Perceived Exertion (RPE) scale runs from 6 to 20, where 6 means no exertion at all and 20 means maximum effort. The numbers in between map to descriptors: 9 is “very light,” 11 is “light,” 13 is “somewhat hard,” 15 is “hard,” 17 is “very hard,” and 19 is “extremely hard.” The scale starts at 6 rather than zero because it was originally designed to roughly correspond to heart rate when you multiply the score by 10. An RPE of 13, for example, loosely tracks with a heart rate around 130 beats per minute in many people.
This sounds imprecise, but perceived exertion correlates strongly with objective markers. In cycling studies, RPE scores tracked closely with both heart rate and blood lactate concentration (a chemical byproduct that builds up as muscles work harder), with correlation coefficients above 0.74. Riders who rated their effort around 12 had lactate levels near 5 to 6 mmol/L, while those rating effort at 16 to 17 had levels above 11 mmol/L, reflecting a genuinely harder physiological state. Your body’s internal sense of effort, in other words, is surprisingly well-calibrated.
To use RPE in practice, check in with yourself at regular intervals during exercise. Pay attention to your breathing, muscle fatigue, and overall sense of strain. For most health-related exercise, you want to stay in the 12 to 14 range (moderate). For performance training, you’ll push into 15 to 17 (hard to very hard) during intervals.
Heart Rate as an Effort Gauge
Heart rate is the most accessible objective measure of physical effort. The American Heart Association defines moderate exercise as 50% to 70% of your maximum heart rate, and vigorous exercise as 70% to 85%. To use these zones, you need an estimate of your maximum heart rate.
The classic formula is 220 minus your age. A more accurate version, developed by researcher Hirofumi Tanaka, uses 208 minus 0.7 times your age. For a 40-year-old, that’s 180 versus 180, so they’re similar at that age. But they diverge for younger and older people. Research on recreational marathon runners found that the 220-minus-age formula underestimates max heart rate in men by about 3 beats per minute and overestimates it in women by about 5 bpm. Tanaka’s formula was more accurate for men, though both formulas slightly overestimated in women.
Neither formula is perfect because individual variation is large. Two 35-year-olds with identical fitness levels can have max heart rates that differ by 10 to 15 bpm. If precision matters to you, a graded exercise test (pushing to true maximum under supervision) gives the real number. For everyday training, the formulas work as rough guides.
Once you know your estimated max, calculating zones is straightforward. If your max is 185, moderate effort falls between 93 and 130 bpm, and vigorous effort spans 130 to 157 bpm. A chest strap heart rate monitor or a wrist-based sensor on a smartwatch can track this in real time.
METs: Measuring the Cost of an Activity
Metabolic equivalents (METs) measure how much energy an activity demands compared to sitting still. One MET equals your resting metabolic rate. Walking at a normal pace costs about 3 METs, meaning your body burns roughly three times the energy it would at rest. The standard thresholds break down like this:
- Sedentary: 1.5 METs or less (sitting, lying down)
- Light intensity: 1.5 to 3.0 METs (slow walking, light housework)
- Moderate intensity: 3.0 to 6.0 METs (brisk walking, cycling at a casual pace)
- Vigorous intensity: 6.0 METs or higher (running, fast cycling, competitive sports)
You don’t need any equipment to use METs. Researchers have cataloged thousands of activities with their MET values, and these tables are freely available online. The practical value is in comparing activities: if you know that gardening is about 3.5 METs and jogging is 7, you understand that jogging demands roughly twice the effort per minute. Health guidelines recommending 150 minutes of moderate activity per week are built on these MET thresholds.
Measuring Mental Effort
Effort isn’t only physical. Mental tasks demand measurable resources too, and several tools exist to quantify cognitive load.
The most widely used subjective tool is the NASA Task Load Index (NASA-TLX), originally developed for cockpit research but now used across dozens of fields. It breaks workload into six dimensions: mental demand, physical demand, time pressure, self-rated performance, effort, and frustration. You rate each on a scale after completing a task, then weight them by importance. The result is a single workload score that captures how taxing the task felt across multiple dimensions, not just “hard” or “easy.”
On the physiological side, your pupils respond to cognitive load in a measurable way. When a task gets harder, your pupils dilate. Researchers call this the task-evoked pupillary response, and studies have confirmed that both peak dilation and mean pupil diameter change significantly between easy and difficult tasks. In one study using an educational video game, peak dilation jumped from a median of 5.1 in easy levels to 18 in hard levels. This makes pupil tracking a useful research tool, though it requires specialized eye-tracking equipment and controlled lighting.
Heart rate variability (HRV) also shifts during mental effort. HRV measures the tiny fluctuations in time between consecutive heartbeats. When you’re mentally engaged or stressed, the variation between beats decreases. In studies comparing easy and hard cognitive tasks, a key HRV metric called RMSSD dropped significantly during harder tasks, with large effect sizes (0.71 to 0.80). Lower HRV during a task means your nervous system is working harder to manage the demand.
How Wearable Devices Track Effort
Consumer wearables have made continuous effort monitoring practical. Most smartwatches and fitness bands use optical sensors on your wrist to detect heart rate, then calculate HRV from the gaps between beats. Different brands measure HRV at different times and using different statistical methods. Apple Watch samples HRV throughout the day and during workouts, reporting a metric called SDNN from one-minute recordings. Fitbit and Oura measure HRV during sleep, using RMSSD over the entire sleep period or in five-minute windows. Whoop tracks HRV during sleep using average intervals between beats.
These devices translate raw HRV data into user-friendly scores. A “strain” score typically reflects how much cardiovascular effort you accumulated during the day, while a “readiness” or “recovery” score uses overnight HRV trends to estimate how prepared your body is for another hard effort. Higher and more consistent HRV from day to day generally indicates better fitness and recovery. A sudden drop in your overnight HRV can signal that yesterday’s effort was greater than your body has recovered from, or that illness, poor sleep, or stress is taking a toll.
The practical limitation is accuracy. Wrist-based optical sensors are less precise than chest straps, especially during high-intensity movement. And because each brand uses its own algorithm, a strain score from one device isn’t directly comparable to another. The trend over weeks and months on your own device matters more than any single-day number.
Combining Methods for Better Accuracy
No single measure captures effort completely. Heart rate tells you about cardiovascular strain but misses local muscle fatigue. RPE captures your overall sensation but is influenced by mood, sleep, and motivation. HRV reflects nervous system load but doesn’t distinguish between physical and emotional stress.
For physical training, pairing RPE with heart rate gives you both the subjective and objective picture. If your heart rate says you’re in a moderate zone but your RPE feels very hard, that mismatch itself is useful information: it could mean you’re fatigued, dehydrated, or coming down with something. For cognitive work, combining a quick NASA-TLX rating with HRV data from a wearable can help you identify which tasks drain you most and when during the day your mental capacity peaks. The most informative measurement of effort, in any context, comes from triangulating at least two sources rather than relying on one alone.