In this guide
The short answer
Give each item on your calendar a rough score for the mental effort it takes, as a share of a full day from 0 to 100. Add the scores up. Keep most days between about 60 and 90%, and when a day goes past 100%, move the heaviest item rather than several small ones.
Why time-based planning overloads you
A calendar measures one thing: time. It treats an hour of errands and an hour of presenting to a client as the same block. They aren’t the same to your brain. One takes almost nothing; the other can leave you useless for the rest of the afternoon.
We’re also bad at predicting how much things will take. Psychologists call it the planning fallacy, a term from Daniel Kahneman and Amos Tversky in 1979. In a 1994 study by Roger Buehler, Dale Griffin and Michael Ross, students estimated they’d finish their senior thesis in about 34 days; on average it took 55, and only about 30% finished on time. When every estimate is a little optimistic and nothing tracks effort, overloaded days sneak up on you.
The cost shows. In Gallup’s 2026 State of the Global Workplace report, 40% of employees worldwide said they had felt a lot of stress the previous day. In the United States and Canada it was 50%, the highest of any region.
What “mental energy” means here
In this guide, mental energy means the effort an activity asks of you: focus, decisions, emotional weight and stakes. It isn’t a medical measurement. It’s your own honest estimate, the same way you’d say a meeting “took a lot out of me.”
If you know spoon theory, Christine Miserandino’s 2003 essay about living with lupus on a limited daily supply of energy, this is a close cousin with numbers: a day has a budget of 100, and everything on it costs some of that budget. Spoon theory belongs to the chronic-illness community that coined it; planning by mental energy borrows the arithmetic, not the name.
Rating your own workload on a 0 to 100 scale isn’t a new idea. NASA’s Task Load Index, developed at NASA Ames Research Center and published by Hart and Staveland in 1988, asks people to rate the mental demand and effort of a task on a 100-point scale. It’s still a standard tool in workload research. Planning by mental energy applies the same kind of self-rating to your day, before it happens instead of after.
How to score your day, in four steps
- List everything, not just work. Meetings and deep work, but also the school run, the dentist and the hard phone call. If it takes something out of you, it counts.
- Give each item a score from 0 to 100, as a share of a full day’s mental energy. Use anchors so your scores stay consistent:
| Activity | Typical score |
|---|---|
| Quick email or message check (15 min) | 2–5 |
| Standard team meeting (1 hour) | 10–18 |
| Focused deep work (2 hours) | 18–28 |
| High-stakes presentation (1 hour) | 25–35 |
| All-day workshop or offsite | 40–60 |
- Add them up. 100 stands for a full day of mental effort. It’s a reference point, not a hard ceiling.
- Adjust before the day starts. Too high? Move one heavy item to a lighter day. Too low? You have room for the thing you’ve been putting off.
Want to try it on a real day? The daily capacity calculator does the math for you.
How much is too much?
Brainwidth uses five levels. They make a good rule of thumb whether you use the app or a notebook:
- Light, under 30%: room to spare. Use it for something important, or rest on purpose.
- Moderate, 30 to 60%: a steady day with slack.
- Good, 60 to 90%: productive without running on fumes. Aim here most days.
- Full, 90 to 120%: little slack left. Past 100%, you’ve planned more than a full day holds.
- Over, 120% and up: well past a full day. Moving the biggest item usually brings it back.
Daily Capacity
Monday · errands and admin
Moderate
52%
Daily Capacity
Thursday · high stakes
Full
114%
One Full or Over day isn’t a problem. Several in a row, with no lighter days between them, is the pattern worth noticing early.
Energy management vs. time management
| Compared on | Time management | Energy management |
|---|---|---|
| The question | When will it happen, and how long will it take? | How much will it take out of me? |
| What it measures | Hours and minutes | Effort, as a share of a full day |
| How it fails | A day that fits on the calendar but not in your head | Scores that drift if you never check them against how days actually felt |
| Best used for | Coordinating with other people | Deciding how much to take on, and what to move |
You need both. Time tells you what fits; energy tells you what’s wise.
Where burnout fits in
The World Health Organization’s 2019 definition describes burn-out as a syndrome “resulting from chronic workplace stress that has not been successfully managed,” marked by exhaustion, mental distance or cynicism about work, and reduced effectiveness. The WHO is clear that it’s an occupational phenomenon, not a medical condition.
Planning by mental energy won’t fix a job that asks too much. What it can do is make overload visible while you can still act on it: before the week starts, not at the end of it. If you’ve felt exhausted for weeks, that’s worth talking to a professional about.
Doing this with Brainwidth
Brainwidth is a planner for iPhone built around this method. It pulls in your Apple and Google calendars and tasks, lets you set each Brainwidth Score with a slider (or suggests one with Auto Estimate, on-device), and shows your daily, weekly and monthly capacity with the same five levels. When a day runs past 100%, it shows the heavy items behind it. See every feature.
Questions
What’s the difference between time management and energy management?
Time management asks when something will happen and how long it takes. Energy management asks how much effort it will cost you. A calendar can be full but manageable, or half empty and exhausting, depending on what’s in it. Planning by mental energy adds that second question to the first.
How do I know what score to give a task?
Rate it as a share of a full day’s mental energy, from 0 to 100. Use anchors: a 15-minute email check is about 2 to 5, a one-hour team meeting 10 to 18, two hours of deep work 18 to 28, and a one-hour high-stakes presentation 25 to 35. Adjust for how draining it is for you specifically.
What’s a healthy daily total?
Aim for roughly 60 to 90% on most days. Under 30% is a light day, which is fine on purpose. Past 100% means you’ve planned more than a full day holds; it’s sometimes unavoidable, but several of those in a row is the pattern to watch.
Do I need an app to plan this way?
No. A notebook works: write each item with a score and add them up. An app like Brainwidth saves the bookkeeping by pulling in your calendar, suggesting scores and showing the total as you plan.
Sources
- Gallup, State of the Global Workplace: 2026 Report (data from 2025)
- World Health Organization, “Burn-out an occupational phenomenon” (2019)
- Buehler, Griffin & Ross, “Exploring the planning fallacy” (1994), Journal of Personality and Social Psychology, 67(3), 366–381
- Hart & Staveland, “Development of NASA-TLX (Task Load Index)” (1988), in Human Mental Workload