Name: Keep the Diff
Author: Lee

Everyone tells you that small decisions add up. Almost nobody puts a number on it, because the number is genuinely hard to see: a purchase you did not make leaves no receipt, no notification and no line in a bank feed.

So here is a month with the number attached. The figures below are ordinary, and the last section is about why the total is smaller than it looks.

One month, written down

Every line is a decision where something was nearly bought. Two numbers each: what was about to be spent, and what was spent instead.

Week	The decision	Ready to spend	Spent	Kept
1	Coffee on the way in, four mornings	$18.00	$0.00	$18.00
1	Friday takeout, cooked instead	$32.00	$9.40	$22.60
1	Second round after work	$14.00	$0.00	$14.00
2	Running shoes, current pair still fine	$129.00	$0.00	$129.00
2	Lunch out, brought leftovers twice	$26.00	$0.00	$26.00
2	Streaming service, canceled after the trial	$12.99	$0.00	$12.99
3	Concert ticket, talked myself out of it	$78.00	$0.00	$78.00
3	Bigger phone plan, stayed on the current one	$15.00	$0.00	$15.00
3	Taxi home, walked half and took the bus	$22.00	$3.20	$18.80
4	Jacket in the tab I closed	$95.00	$0.00	$95.00
4	Groceries, store brand across the shop	$88.00	$71.50	$16.50
4	Repaired the bike instead of replacing it	$240.00	$45.00	$195.00

Twelve decisions. Four of them were partial: money was spent, just less of it. The total kept is $640.89.

What that number means

Nothing was transferred to a savings account. The $640.89 is the difference between what a month would have cost on the default path and what it actually cost.

That distinction matters, because the honest version of this exercise has to deal with a real objection.

The objection: you were never going to buy some of these

The objection is the strongest criticism of counting decisions, and it is correct.

The $240 bike is the obvious one. Was replacing it ever seriously on the table, or does it appear on the list because writing down $195 felt good? Nobody can audit that but you, and the temptation to inflate the planned figure is exactly what makes a total like this stop being believable.

One rule keeps it honest: write what you had actually accepted paying. If you were choosing between a $240 replacement and a $45 repair, the planned figure is $240, because that was a real option you were weighing. If you saw a $2,000 bike online and closed the tab, that is not a $2,000 decision. That $2,000 was never yours to spend.

Apply that rule and the number stays defensible. Ignore it and you have invented a scoreboard, which you will stop believing within two weeks and then stop using.

Where the total actually comes from

Look at where the $640.89 sits. Two decisions, the bike repair and the jacket, are $290.00 of it, which is 45 percent of the month. The six entries under twenty dollars come to $95.29 between them.

The usual description gets this backward. The standard advice is about skipping small daily purchases, and the small daily purchases are the minority of the money here. The infrequent, larger decisions are where the total is made, and they are also the ones you make consciously enough to record.

So if writing down every coffee sounds exhausting, skip the coffees. Catch the decisions over about twenty dollars and you will capture most of the number with a fraction of the effort.

What a typical month looks like

Twelve entries in a month is roughly three a week, which is a realistic rate for somebody paying attention without turning it into a hobby. The total will swing a lot: a month with a repair-or-replace decision in it looks very different from one without.

What tends to be stable is the shape. Mostly small entries, a couple of large ones, and a few partial decisions that a pass-or-fail tracker would have thrown away entirely. In this month the four partial entries alone are $252.90, which is 39 percent of the total, and every one of them would have scored as a failed day under a no spend challenge.

At this pace: turning a month into a year

The tempting next step is to multiply. $640.89 a month is $7,690 a year, and that figure feels wonderful. It is also the number most likely to mislead you, because it assumes every month contains a bike repair.

An honest projection takes a little more care. Split the month into its four weeks:

Week 1: $54.60
Week 2: $167.99
Week 3: $111.80
Week 4: $306.50

Week 4 carries the bike and the jacket, and it is more than five times week 1. Use the typical week instead of the average one: halfway between the two middle weeks, which here is $139.89. Multiply by 52 and round down, and the honest pace is about $7,274 a year. That is still a large number, and it is one you can believe.

Three rules keep a projection like this from flattering you:

Wait for four full weeks. A projection from three good days is a guess.
Count the quiet weeks. A week where you logged nothing counts as zero. Leaving it out makes every other week look like the norm.
Round down. Understating your pace costs you nothing. Overstating it makes the next real month feel like a failure.

Keep the Diff works out its "At this pace" line exactly this way. It appears after four complete weeks, uses your typical week, counts the empty ones, and rounds down. With Premium, each goal also shows roughly when you will reach it at the same pace.

What to do with the number

Two options, and only one of them is really a savings method.

You can treat it as information. Knowing that a normal month contains six hundred dollars of decisions changes how the next one feels, and that alone is worth the ten seconds an entry takes.

Or you can make it real by moving the money. At the end of the week, transfer what the total says into savings. The transfer turns a record into a balance, and it is the only way the number stops being notional. It also introduces a limit the record does not have, which is whether the money is actually there to move.

If you want to know how long a total like this takes to reach something specific, the calculator does that division: at $160 a week, which is what this month averages, a $2,400 goal is fifteen weeks.

Try it on your own month

The exercise is worth doing once whether or not you keep it up, because most people have never seen the figure. Use the tracker in your browser, a printed sheet, or a note on your phone. Two numbers per decision, the real ones, for four weeks.

Keep the Diff does the same thing with the adding up handled for you, free on iPhone, iPad, Mac, Apple Watch, Windows, Linux and Android. The reasoning behind counting money is in a separate post.

How much do you actually save by not buying?