If I say I'm saving $10 a week, then multiple that by 4 which equates $40 per month. Multiple that by 12 for each month in the year, that's $480 annually. But if say $10 multiplied by 52, that's $520 annually. How much am I actually saving? Eli5 the difference!

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breckenridgeback t1_jaaj7zv wrote on February 28, 2023 at 1:05 AM

#2,083,169

There are not 4 weeks in a month. 4 weeks is 28 days, which is a bit less than every month except a non-leap-year February.

A closer estimate would be 4.5 weeks in a month, which would be $45 a month times $12 = $540, fairly close to your (correct) $520 computed by just using the number of weeks in a year. (Although, strictly speaking, there are 52 weeks + 1 or 2 days in a year, depending on whether it's a leap year.)

mdotca t1_jaajbin wrote on February 28, 2023 at 1:06 AM

#2,083,196

4x12 is not 52. You’ve made an assumption about the relationship between months and weeks. There are 52.18 weeks in a year.

TapataZapata t1_jaajepw wrote on February 28, 2023 at 1:06 AM

#2,083,223

A month does have *roughly* four weeks, not exactly. Take a look at a calendar and see for yourself. Or do the math with the number of days: a week has always 7 days. A month has 28, 29, 30 or 31 days. 28 is 4 weeks, 31 is more.

[deleted] t1_jaali5r wrote on February 28, 2023 at 1:21 AM

#2,083,798

[removed]

jensjoy t1_jaamhe5 wrote on February 28, 2023 at 1:29 AM

#2,084,084

>then multiple that by 4 which equates $40 per month

You've got a wrong premise, leading to the difference you noticed. Apart from February, four weeks (28 days) do not equal one month (30/31 days).

pineapple_and_olive t1_jaatdka wrote on February 28, 2023 at 2:20 AM

#2,086,105

Replying to mdotca (#2,083,196)

Just 365x your savings per day.

Forget weeks and months.

Severe_Atmosphere_44 t1_jaausul wrote on February 28, 2023 at 2:30 AM

#2,086,513

So $10 × 52 weeks = $520 annually. $520 ÷12 = $43.33 per month. $520 ÷ 365 = $1.43 per day.

Estimates are more accurate by figuring annually first, then dividing by the smaller units.

urzu_seven t1_jabb5qy wrote on February 28, 2023 at 4:44 AM

#2,091,367

The difference is you made a false assumption that 1 month = 4 weeks. This is only true for ONE month (most of the time, hi leap years!) February. Every single other month has 2-3 more days which makes up the difference.

Jan: 4 w + 3 d
Feb: 4 w + 0 d (1 d in leap years)
Mar: 4 w + 3 d
Apr: 4 w + 2 d
May: 4 w + 3 d
Jun: 4 w + 2 d
Jul: 4 w + 3 d
Aug: 4 w + 3 d
Sep: 4 w + 2 d
Oct: 4 w + 3 d
Nov: 4 w + 2 d
Dec: 4 w + 3 d

Totals: 48 w + 29 or 30 d = 52 w + 1 or 2 d