Everyday
Jet Lag
Jet Lag solves the little math problems of daily life: dates, trips, timers and quick estimates.
What this does
Jet Lag solves the little math problems of daily life: dates, trips, timers and quick estimates.
Everyday math is the arithmetic of daily life: tips, bill splits, discounts, unit prices, and quick percentages. None of it is hard, but all of it happens under mild time pressure at a table, a register, or in an aisle, which is where the mistakes creep in. These calculators exist to give you the right number in seconds so the social moment stays pleasant and the money stays fair.
The formula, explained plainly
Tip math is a percentage of the pre-tax subtotal in most US convention (15 to 20 percent standard, more for exceptional service), though many people tip on the total out of simplicity. The quick mental method: 10 percent is moving the decimal one place, double it for 20 percent, halve the 10 percent for 5 percent, and combine. Splitting a bill fairly means dividing the subtotal plus tax and tip proportionally, not just splitting the total evenly when one person ordered the steak and another the salad.
Discount math has one reliable shortcut: a 25 percent discount means you pay 75 percent, so multiply the price by 0.75. Stacked discounts do not add: 20 percent off followed by another 10 percent off is not 30 percent off, it is 0.8 times 0.9 = 0.72, or 28 percent off. "Buy one get one half off" is 25 percent off two items, and "buy two get one free" is 33 percent off three. Retailers count on shoppers adding instead of multiplying.
Unit price math is division that defeats packaging psychology: price divided by quantity (weight, volume, or count) gives the true per-unit cost. The bigger package is usually but not always cheaper per unit; "value packs" sometimes cost more per ounce than the regular size, and the unit price on the shelf tag is the fastest truth available. Comparing across different units (per ounce vs per pound) just needs one conversion first.
Quick percentage fluency covers the rest: percentage change is (new minus old) divided by old, percentage points are the arithmetic difference between two percentages (a rise from 10% to 15% is 5 percentage points but a 50 percent increase), and "percent of" is just multiplication by the decimal. These three distinctions resolve most everyday number confusion, from sale signs to news statistics.
How to use it
- Enter time zones crossed.
- Read the instant result and the breakdown below it.
- Adjust any input to compare scenarios.
Worked example
With time zones crossed = 6, the result is 4 days to adjust. Defaults demonstrate the most common use.
Common mistakes
- Using the sale price as the base for 'percent saved' instead of the original price, which understates or overstates the deal.
- Rounding the bill up 'for simplicity' on every shared expense, when a year's worth of casual round-ups is real money.
- Doing percentage change backward (dividing by the new value instead of the old), which gives a wrong 'discount percent' on every sale tag you check.
- Tipping on the post-tax total versus pre-tax subtotal without realizing it, which adds a couple percent either way and causes table debates.
- Splitting the bill evenly when orders differed wildly, quietly overcharging the light eaters and subsidizing the big spenders.
- Adding stacked discounts (20% + 10% = 30%) instead of multiplying (0.8 x 0.9 = 28% off), which is exactly the error sale signs hope you make.
- Forgetting tax when mental-math budgeting a purchase, then being surprised at the register by the 6 to 10 percent that was never in the plan.
- Comparing package sizes by total price instead of unit price, falling for the 'value pack' that costs more per ounce.
Limitations
- Tip conventions vary by country and context; the calculator defaults to US restaurant norms but tipping customs differ widely worldwide.
- Bill-split math divides money, not goodwill: it cannot settle who ordered what from memory alone, so check the itemized receipt for accuracy.
- Discount calculations use the prices you enter; they do not verify the 'original' price was ever real, and inflated was-prices are a common retail tactic.
- Unit price comparisons assume identical products; quality, freshness, and waste (the giant tub that spoils) can make the cheaper unit price the worse buy.
- Tax rates entered are your responsibility; combined state, local, and special district rates vary by exact address.
Expected accuracy
Everyday arithmetic is exact: percentages, splits, and unit prices compute precisely from your inputs. Real-world accuracy is about inputs: the receipt's subtotal, the shelf's unit price, the actual tax rate. Enter the receipt numbers and the answer is correct to the cent.
Privacy
Everything you type stays on your device. The calculation runs in your browser with JavaScript; no input is sent to a server, stored in an account, or shared with anyone.
Sources and standards
- Standard percentage and proportional arithmetic; tip conventions follow common US restaurant practice (15-20% on pre-tax subtotal); unit pricing follows the price-per-quantity comparison used on retail shelf labels.
Bottom line
Jet Lag settles the table math in seconds, fairly and without awkwardness. Enter the receipt as it is, pick the convention you want, and let everyone pay exactly their share.
Key insight
Estimate before you calculate. A rough mental answer first turns the calculator into a checker instead of an oracle, and you will catch input typos instantly.
Frequently asked questions
How should we split shared costs like a vacation rental?
Proportionally to use: nights stayed, bedroom size, or headcount, agreed in advance. Equal splits suit equal use; anything else deserves the proportional math. Put the rule in writing before booking to avoid the end-of-trip negotiation.
How do I figure out how much I saved on sale?
Subtract sale price from original price for dollars saved; divide that by the original price for percent saved. Saving 24 on an 80 item (30%) feels different from saving 24 on a 400 item (6%), which is why both numbers matter.
What is a fair tip for delivery or takeout?
Conventions vary: 10-15% for delivery (more in bad weather or long distances), and a few dollars or 10% for takeout where you collected it yourself. The calculator covers all three modes so you can tip consistently without overthinking each order.
How do I calculate a 20% tip quickly?
Move the decimal one place left for 10%, then double it. On a 46.50 bill, 10% is 4.65, so 20% is 9.30. For 15%, take the 10% and add half again (4.65 + 2.33 = 6.98). The calculator does it exactly, but the mental method keeps you sharp.
Should I tip on the pre-tax or post-tax amount?
US convention is the pre-tax subtotal, with 15-20% standard for table service. Many people tip on the total for simplicity, which adds roughly the tax rate extra. Either is socially fine; the calculator lets you choose the base so the table agrees on one method.
How do I split a bill fairly when orders differ?
Divide each person's subtotal items, allocate tax and tip proportionally to subtotals, and sum per person. If one person's subtotal is 40% of the bill, they pay 40% of the tax and tip. Even splits are only fair when orders were roughly even.
How do stacked discounts really work?
Multiply the remaining fractions: 20% off then 10% off is 0.80 x 0.90 = 0.72, so you pay 72% (28% off), not 70%. Each discount applies to the already-reduced price. The more discounts stack, the bigger the gap between adding and the correct multiplication.
What does 'buy one get one 50% off' actually save?
25% off the pair: you pay full price plus half price, which is 1.5 items' worth for 2 items. 'Buy two get one free' is 33% off three items. Convert every promo to 'pay X for Y' and the real discount is obvious.
How do I find the better deal between two package sizes?
Divide each price by its quantity in the same unit to get unit price, then compare. A 500 g box at 4.00 (0.80/100 g) beats a 1 kg 'value' box at 9.00 (0.90/100 g). Watch for different units on the shelf tags and convert first.
How do I calculate percentage change?
Subtract old from new, divide by old, multiply by 100. A price rising from 40 to 50 is (50-40)/40 = 25% increase. Always divide by the starting value; dividing by the new value answers a different question and gives a wrong 'discount' figure.
What is the difference between percent and percentage points?
Percentage points are the arithmetic gap: 10% to 15% is 5 percentage points. Percent change is relative: that same move is a 50% increase. News and contracts exploit the confusion; the calculator shows both so you know which one you are looking at.
How do I add tax to a price mentally?
Convert the tax rate to a decimal and multiply: at 8% tax, a 50 item costs 50 x 1.08 = 54. For quick estimates, round the rate (8% ~ 10% minus a bit). The calculator uses your exact local rate so the register total holds no surprises.
How do I split the tip when service was shared?
Apply the same tip percentage to the whole bill, then divide each person's total (subtotal share + proportional tax + proportional tip). If you are splitting evenly by agreement, just divide the grand total by headcount. The key is agreeing on the method before the bill arrives.