Physical activity can be measured in several ways, ranging from a simple speech test you can do mid-workout to laboratory techniques that track every calorie your body burns. The right method depends on what you’re trying to learn: whether you’re hitting weekly health targets, training at the correct intensity, or tracking progress over time. Here’s how each approach works and when to use it.
The Talk Test: Simplest Measure of Intensity
The quickest way to gauge how hard you’re working requires no equipment at all. The CDC recommends a straightforward rule of thumb: if you can carry on a conversation but couldn’t sing a song, you’re at moderate intensity. If you can’t get out more than a few words without stopping for a breath, you’re in vigorous territory. This is surprisingly useful for everyday exercise because the WHO recommends adults get at least 150 minutes of moderate-intensity activity per week, and the talk test lets you confirm you’re in that zone without any gadgets.
Rating Perceived Exertion
The Borg Rated Perceived Exertion (RPE) scale puts a number on how hard exercise feels. It runs from 6 (no exertion, sitting still) to 20 (absolute maximum effort). A score of 11 to 12 corresponds to “light” effort, 13 feels “somewhat hard,” 15 is “hard,” and anything above 17 is “very hard.” The scale was designed so that multiplying your rating by 10 roughly approximates your heart rate, though that relationship is loose.
RPE is useful when you don’t have a heart rate monitor or when heart rate isn’t reliable, such as during strength training, swimming, or if you take medications that affect heart rate. You simply check in with yourself periodically and assign a number. Over time, tracking RPE helps you notice when a workout that once felt like a 15 now registers as a 12, which is a concrete sign of improved fitness.
Measuring Intensity With Heart Rate
Heart rate gives you an objective, real-time number for how hard your cardiovascular system is working. The basic formula for estimating your maximum heart rate is 220 minus your age. A 40-year-old, for example, would have an estimated max of 180 beats per minute. For moderate-intensity exercise, aim for 64% to 76% of that maximum. For higher-intensity work, the target range is 77% to 93%.
A more personalized approach is the Karvonen method, which factors in your resting heart rate. You subtract your resting heart rate from your maximum heart rate to get your “heart rate reserve,” then multiply that reserve by your target percentage and add your resting heart rate back in. If that same 40-year-old has a resting heart rate of 65, their heart rate reserve is 115. To exercise at 70% intensity, the target would be (115 × 0.70) + 65, which equals about 146 beats per minute. This method is more accurate for people whose resting heart rate is unusually high or low.
METs: Comparing Activities by Energy Cost
A MET, or metabolic equivalent, is a standardized unit for comparing the energy cost of different activities. One MET is the energy you burn at complete rest, roughly equivalent to quiet sitting or sleeping. Every activity gets a MET value based on how many times harder it is than rest. Walking at a moderate pace (about 3 mph) is rated at 3.5 METs. General cycling comes in at 7.5 METs. Running at 10 mph scores 14.5 METs, meaning it burns about 14.5 times the energy of sitting still.
METs are especially handy for comparing activities that feel different but burn similar energy, or for calculating weekly activity volume. Many health guidelines define moderate activity as 3.0 to 5.9 METs and vigorous activity as 6.0 METs or above. You can also multiply METs by minutes to get “MET-minutes,” a single number that captures both intensity and duration. If you walk at 3.5 METs for 30 minutes, that’s 105 MET-minutes. The WHO’s 150-minute moderate guideline translates to roughly 500 to 1,000 MET-minutes per week.
Step Counts, Pedometers, and Accelerometers
Pedometers are the simplest motion-tracking devices. They contain a small lever arm that moves up and down in response to the movement at your hip with each step, giving you a raw step count. They’re cheap and easy to use, but they tell you nothing about intensity or calories.
Accelerometers, the sensors inside smartphones and wrist-worn fitness trackers, are more sophisticated. They measure acceleration in three directions: side to side, up and down, and front to back. Software algorithms then translate that raw motion data into estimated steps, distance, and calories burned. The tradeoff is that accelerometers still struggle with activities where your wrist or hip doesn’t move much. They can’t accurately capture cycling, weight lifting, yoga, or any seated exercise. If those activities make up a significant part of your routine, an accelerometer-based tracker will undercount your total activity.
For walking and running, though, wearable trackers are a practical daily measurement tool. Pairing step counts with heart rate data from an optical sensor (as most modern wearables do) gives a more complete picture than either metric alone.
Measuring Strength Training Volume
Aerobic metrics like heart rate and steps don’t translate well to resistance training. The standard way to quantify weight training is volume load: weight multiplied by sets multiplied by reps. If you squat 100 kg for 5 sets of 5 reps, your volume load for that exercise is 2,500 kg. Tracking this number across weeks lets you see progressive overload in concrete terms. A jump from 2,500 kg to 3,000 kg means you did more total work, whether that came from heavier weight, more reps, or additional sets.
Volume load has limits. It doesn’t account for how close each set was to failure, how long you rested between sets, or the range of motion you used. But as a simple, repeatable measure of how much you lifted, it’s the most widely used number in strength training.
Questionnaires and Self-Report Tools
When researchers need to measure physical activity across large populations, they often use validated questionnaires. The International Physical Activity Questionnaire (IPAQ) is one of the most common. Its short form has just 8 items and estimates the time you spend in moderate-to-vigorous activity plus the time you spend sitting. It was designed for surveillance studies where detailed measurement isn’t feasible.
Self-report tools are convenient but consistently overestimate activity levels. People tend to remember their exercise sessions but forget how much time they spent sitting. For personal tracking, a questionnaire can be a useful starting point if you have no other tools, but pairing it with even a basic pedometer gives a more honest picture.
Physical Activity Level (PAL) Scores
Your Physical Activity Level is a single number that describes how active your entire day is, not just your exercise sessions. It’s calculated by dividing your total daily energy expenditure by your resting energy expenditure. A PAL between 1.40 and 1.69 indicates a sedentary or lightly active lifestyle. A range of 1.70 to 1.99 reflects a moderately active lifestyle. Vigorously active people, such as manual laborers or serious athletes, fall between 2.00 and 2.40.
PAL is useful because it captures everything: your commute, your job, household chores, and formal exercise combined. Someone who runs for 30 minutes a day but sits for the remaining 15 waking hours may have a lower PAL than a mail carrier who never sets foot in a gym.
The Laboratory Gold Standard
The most accurate measurement of total energy expenditure in free-living humans is the doubly labeled water technique. You drink a small dose of water containing two rare but stable isotopes: a heavy form of hydrogen and a heavy form of oxygen. Over the following days, both isotopes leave your body, but through different pathways. The hydrogen isotope exits only as water (sweat, urine, breath moisture), while the oxygen isotope exits as both water and carbon dioxide. The difference between the two elimination rates reveals exactly how much carbon dioxide your body produced, which can then be converted into a precise estimate of total calories burned.
This method captures every calorie from every activity over a one- to two-week period, including fidgeting, digestion, and sleep. It’s the benchmark researchers use to validate cheaper tools like accelerometers and questionnaires. It’s also expensive and requires isotope analysis in a lab, so it’s not something you’d use for personal fitness tracking. But when a study claims a wearable is “X% accurate,” doubly labeled water is almost always the reference it’s being compared against.