Spending Calculators

Three spending questions everyone runs into

Only three calculators live on this shelf, because everyday spending really comes down to three questions. Where should the month's money go, which the Budget Calculator answers with the 50/30/20 method. What does that percentage on the sale tag actually cost me, which the Percent Off Calculator answers in one clean multiplication. And what happens when the pricing gets complicated, a voucher on top of a markdown, a "was" price that needs checking, two codes stacked at the checkout, which is the Discount Calculator's territory. Each tool is free, and each shows the working rather than just the answer, so the method sticks after the tab is closed.

The 50/30/20 budget, and what it actually tells you

The 50/30/20 rule has a citable origin: the 2005 book "All Your Worth" by Elizabeth Warren and Amelia Warren Tyagi. Their proposal splits take-home pay three ways, roughly half to needs, under a third to wants, and a fifth to savings plus any debt repayment beyond the minimums already due. It caught on because it needs no spreadsheet, just three buckets and a percentage each. The Budget Calculator does exactly that arithmetic: every line of spending you enter is tagged to a bucket, each bucket is shown as a percentage of income against its guideline, and whatever no bucket claims becomes the leftover figure, which can run negative if the rows outgrow the income. The income box wants monthly take-home pay; if all you know is an annual salary, the Salary Calculator converts it first.

Needs against wants, honestly drawn

Rent, groceries, utilities, minimum debt payments and getting to work are needs by any reasonable reading; a second streaming service is a want however automatic it feels on the statement. The line genuinely blurs in the middle, a car is a need where there is no public transport and a want where there is a good one, and no calculator can make that judgement for you. The practical test the page suggests is worth keeping: if cancelling something for a month would cause a real problem, it is a need; if it would only cause disappointment, it is a want.

The third bucket, and paying yourself first

Savings & debt covers money actually saved plus anything paid onto a card or loan beyond the minimum due, because both are choices about the future rather than bills owed this month. Treating that bucket as a fixed line decided before the month starts, rather than whatever survives to the end of it, is the habit usually called paying yourself first. To see what a chosen monthly amount actually builds toward, and by when, carry the figure to the Savings Calculator; to see what the same money does over decades rather than months, the Compound Interest Calculator makes the long version of the argument.

Annual costs, and when 50% is impossible

Two failure modes catch most budgets. The first is the once-a-year bill: car insurance or an annual subscription that a monthly budget quietly forgets for eleven months. The fix is arithmetic, not willpower: divide the annual figure by 12 and enter it as its own monthly row, so a £480 policy becomes a £40 line that is never a surprise. The second is treating 50/30/20 as a law. In expensive housing markets, rent alone can pass half of take-home pay, which makes the 50% needs line mathematically unreachable rather than a matter of discipline. The useful reading then is the gap itself, how far over and why, not a pass or fail grade against percentages written in 2005.

Percent off: one multiplication, done properly

A percentage off is a share removed from the price, so what you pay is the share that survives: pay = price × (1 − percent ÷ 100). Taking 20% off means keeping 80%, and multiplying £50 by 0.8 lands on £40 in one step. The Percent Off Calculator does that sum for any price and rate, states the saving in cash as well as the final price, and charts the same price at every discount step from 5% to 90%, so you can see exactly what moving up one sale tier is worth.

It also teaches the head-maths worth keeping. Ten percent of any price is the decimal point moved one place left, and every common rate is built from that anchor: double it for 20%, treble it for 30%, halve it for 5%. A quarter off is easiest as two halvings. And sale language in fractions maps straight onto rates, a third off is 33.3%, quarter price means 75% off. The page's other lesson is about the signs themselves: "up to 70% off" is a ceiling, not an average, the single largest markdown anywhere in the sale, and a displayed "was" price anchors your sense of the deal whether or not it was ever the common selling price. The percentage is only as informative as the reference price behind it, which is why the cash saving deserves equal billing.

Stacked discounts, vouchers and working backwards

The Discount Calculator picks up where a single clean percentage stops. Its three modes match the three shapes the question actually arrives in: a percentage off a price, a flat amount off, and working back to the original price from a sale price you can see.

Why 20% plus 30% is not 50%

Stacked discounts multiply rather than add, because the second discount applies to the already-reduced price, never to the original. A $100 item marked down 20% falls to $80; an extra 30% at the till comes off that $80, removing $24 rather than $30, for a final price of $56. The combined discount is 44%, not 50%, and the gap grows with the size of the rates, which is precisely why "stack these codes" offers are more conservative than they sound. The order of the two discounts makes no difference, since multiplying by 0.8 then 0.7 equals multiplying by 0.7 then 0.8; what matters is that each one is calculated from the price immediately before it.

Undoing a discount means dividing

Working back from a sale price is the same trap in reverse. A 40% discount was calculated from the original price, so adding 40% of the sale price back on is not enough to undo it. Divide instead: original = sale price ÷ (1 − rate). A jacket at $45 after 40% off was 45 ÷ 0.6 = $75 originally, not the $63 that adding 40% back would suggest. The same divide-not-subtract logic governs pulling tax out of a receipt total, which is why the Sales Tax Calculator and this tool make natural companions on the same receipt. Flat-amount offers round out the set: a £20 voucher on an £80 order is a 25% effective discount, and converting a voucher to its effective rate is the only fair way to compare it against a percentage offer on the same basket. For percentage questions beyond shopping entirely, the Percentage Calculator is the general-purpose tool.

A percentage is not an amount of money

The thread running through all three tools is that a percentage means nothing without the number it is taken from. Fifty percent off a $20 item saves $10; twenty percent off a $200 item saves $40, four times the cash from a headline that sounds less than half as generous. The same blindness works on budgets: a category that doubles from 2% to 4% of income matters less than one that creeps from 30% to 33%. All three calculators therefore insist on showing cash figures next to every rate, the saving in money on the discount tools and the pound amounts behind every bucket percentage in the budget. Comparing offers, or months, by percentage alone is the mistake this shelf exists to end.

Using the three together

A real sale season runs through all three tools in order. The Percent Off Calculator reads a single tag: what does 30% off this price actually cost and save? The Discount Calculator handles the checkout: the loyalty voucher on top of the markdown, and the receipt check afterwards to confirm the "was" price was real. And the Budget Calculator answers the question the till never asks: whichever bucket this purchase belongs to, does it fit? A discount is only ever money saved if the purchase was going to happen anyway; 44% off something that was never in the plan is 56% spent, and the budget page is where that shows up honestly.

Questions people ask

Is 20% off followed by 10% off the same as 30% off?

No, it is always slightly less. The second discount applies to the already-reduced price, so 20% then 10% combines to 28% off. Only a single 30% discount applied once gives exactly 30%.

How do I find the original price from a sale price?

Divide the sale price by one minus the discount rate as a decimal. A £60 sale price after 25% off was 60 ÷ 0.75 = £80 originally. Adding the percentage back onto the sale price undershoots the true original every time.

Is 50/30/20 a strict rule I have to hit?

No, it is a starting guideline from a 2005 book, not a law. High housing costs can put the 50% needs line out of reach through no budgeting fault at all; the useful output is the size of each gap and what is driving it, not a pass or fail verdict.

Which tool handles a fixed-amount voucher?

The Discount Calculator, in its amount-off mode. It subtracts the flat amount and reports the effective percentage, so a £20 voucher on £80 shows up as 25%, directly comparable with any percentage offer.

Why does the same percentage save different amounts?

Because a percentage is relative to the price it is taken from. Twenty percent of $20 is $4 and twenty percent of $200 is $40, so comparing two offers fairly means looking at the cash saved and the final price paid, not the headline rates alone.