Work out the latest day you can start.

The total stays the same. The days change.

Waiting doesn't make the work any smaller. It leaves fewer days to divide it by. The calculator adds up your parts, multiplies them by what your last estimate taught you, and divides by the working days left, for every day you could start. The latest start is the last day where that comes out at or under your own limit.

For example, 20 hours over 10 working days is 2 hours a day. Over 5 days it's 4 hours, and over 2 days it's 10.

Each day you let go costs more than the one before, because the same total is divided by a smaller number each time. That is why the curve gets steeper the closer it gets to the due date.

For an exam, the parts are your revision: going through your study material, practice questions, past papers. How many days before the exam to start comes from the same sum: the hours, divided by the most you'll do in a day, rounded up and counted back in working days from the day before the exam.

Your estimate is probably short

People tend to underestimate how long their own work will take. In one study, psychology students writing an honours thesis predicted, on average, that they would finish it in 33.9 days. They took 55.5, and fewer than a third finished by the date they gave. Asked for a worst case, they said 48.6 days on average, and fewer than half had finished by then (Buehler, Griffin and Ross, 1994).

Two things helped in the research. Listing a task's parts before estimating it gave longer estimates, and in two of the experiments less biased ones; the more parts a task had, the bigger the difference (Kruger and Evans, 2004). And in a later study in the Buehler paper, the optimism went away for people who were asked to connect their prediction to how similar tasks had gone before. That is why the calculator asks for parts, and asks how your last estimate went. There is no standard multiplier; the right one is whatever happened to you.

What research says about putting things off

Two studies that followed students through a term found that those who put things off reported less stress and illness than the others early in the term. Late in the term they reported more of both, they were sicker overall, and their grades were lower on all their assignments (Tice and Baumeister, 1997).

A review that pooled 33 studies with 38,529 participants found the same direction: more procrastination went with lower academic performance. How clearly the link showed depended on how it was measured, and studies that relied on self-report scales had more trouble detecting it (Kim and Seo, 2015).

These are averages across groups of students. They describe what tends to happen, not what will happen to you.

If the plan needs a late night

A plan that only works with a night without sleep has a cost this calculator can't count. After 24 hours awake, college students did worse on a critical-thinking test than students who had slept about eight hours, yet rated their own concentration and effort higher and thought they had done better (Pilcher and Walters, 1997).

What an all-nighter actually costs you goes through that trade in more detail, and sleep is part of studying covers what sleep does for what you've already learned.

Start with one small step

A plan that says when and where you'll act, in the form “if it's this time and I'm in this place, then I'll do this”, is called an implementation intention. Across 94 tests, forming one had a medium-to-large effect on whether people reached their goals, d = 0.65 (Gollwitzer and Sheeran, 2006). That is what the last card in your result is for: a day, a time, a place and 25 minutes.

Twenty-five minutes is one Pomodoro. The post on the Pomodoro technique covers what those blocks are good for and where they get in the way.

Sources

1. Buehler, R., Griffin, D. and Ross, M. (1994). Exploring the "planning fallacy": why people underestimate their task completion times. Journal of Personality and Social Psychology, 67(3), 366–381.
2. Kruger, J. and Evans, M. (2004). If you don't want to be late, enumerate: unpacking reduces the planning fallacy. Journal of Experimental Social Psychology, 40(5), 586–598.
3. Tice, D. M. and Baumeister, R. F. (1997). Longitudinal study of procrastination, performance, stress, and health: the costs and benefits of dawdling. Psychological Science, 8(6), 454–458.
4. Kim, K. R. and Seo, E. H. (2015). The relationship between procrastination and academic performance: a meta-analysis. Personality and Individual Differences, 82, 26–33.
5. Pilcher, J. J. and Walters, A. S. (1997). How sleep deprivation affects psychological variables related to college students' cognitive performance. Journal of American College Health, 46(3), 121–126.
6. Gollwitzer, P. M. and Sheeran, P. (2006). Implementation intentions and goal achievement: a meta-analysis of effects and processes. Advances in Experimental Social Psychology, 38, 69–119.