The news says inflation is running at some number. Then you check out at the grocery store and the total feels like it's from a different country. You're not imagining it, and you're not necessarily wrong. The official number measures an average household. You aren't one.
Your personal inflation rate is how much more your regular purchases cost now than they did before. It's not hard to calculate, and your receipts already hold everything you need.
CPI vs. your basket
The headline inflation figure in the US usually refers to the Consumer Price Index, or CPI, published monthly by the Bureau of Labor Statistics. CPI tracks the prices of a broad basket of goods and services — food, housing, transportation, medical care, clothing, recreation and much more — and weights each piece by how much households spend on it on average.
That's a fine way to measure the economy. It's a rough way to measure your life, because your basket isn't the average basket:
- If you don't drive, gas prices barely touch you. If you commute 60 miles a day, they hit hard.
- If you rent, your rent increase matters far more to you than to the national average. If you own with a fixed-rate mortgage, your housing payment may not move at all.
- If you drink a lot of coffee or cook with olive oil every day, a jump in those prices shows up in your budget much more than in an index where they're a tiny slice.
So two neighbors can live through the same year and see very different inflation. One notices nothing; the other feels squeezed every week. Both are right about their own basket.
This article focuses on the part you can measure most easily and change most directly: everyday shopping, especially groceries. The same method works for anything you buy repeatedly.
The formula
The idea is simple. For each item, find how much its price changed. Then weight that change by how much of your spending the item takes up. An item you spend a lot on should count for more than one you buy once in a while.
For each item:
- Price change = (new price − old price) ÷ old price
- Share of spend = what you spent on that item ÷ what you spent on the whole basket (using old prices)
- Contribution = price change × share of spend
Then add up the contributions. The total is your personal inflation rate for that basket.
There's an equivalent shortcut that's easier to check: price the same basket, in the same quantities, at old prices and at new prices.
Personal inflation = (basket cost at new prices − basket cost at old prices) ÷ basket cost at old prices
Both methods give the same answer, as long as you keep quantities fixed. That's the key rule: you're measuring prices, not how much you bought. If you started buying twice as much coffee, your spending went up, but coffee didn't get more expensive.
A worked example
Say you pull a month of grocery receipts from a year ago and a month from now. You pick eight things you buy regularly and hold the quantities the same in both months.
| Item | Monthly qty | Old price | New price | Change | Old spend | Share | Contribution |
|---|---|---|---|---|---|---|---|
| Eggs (dozen) | 4 | $3.00 | $3.60 | +20% | $12.00 | 8.0% | +1.60 pts |
| Milk (gallon) | 6 | $3.50 | $3.85 | +10% | $21.00 | 14.0% | +1.40 pts |
| Coffee (12 oz bag) | 2 | $9.00 | $10.80 | +20% | $18.00 | 12.0% | +2.40 pts |
| Chicken breast (lb) | 10 | $4.00 | $4.20 | +5% | $40.00 | 26.7% | +1.33 pts |
| Bread (loaf) | 4 | $3.00 | $3.00 | 0% | $12.00 | 8.0% | 0.00 pts |
| Bananas (lb) | 10 | $0.60 | $0.57 | −5% | $6.00 | 4.0% | −0.20 pts |
| Cereal (box) | 3 | $5.00 | $4.50 | −10% | $15.00 | 10.0% | −1.00 pts |
| Olive oil (bottle) | 2 | $13.00 | $14.95 | +15% | $26.00 | 17.3% | +2.60 pts |
| Total | $150.00 | 100% | +8.13 pts |
Step by step:
- Old basket cost. Multiply quantity by old price for each row and add them up: 12 + 21 + 18 + 40 + 12 + 6 + 15 + 26 = $150.00.
- Shares. Divide each row by $150. Chicken is $40 ÷ $150 = 26.7% of the basket. Olive oil is $26 ÷ $150 = 17.3%.
- Contributions. Multiply each share by the price change. Olive oil: 17.3% × 15% = 2.60 points. Chicken: 26.7% × 5% = 1.33 points. Cereal: 10% × (−10%) = −1.00 point.
- Add them up. 1.60 + 1.40 + 2.40 + 1.33 + 0 − 0.20 − 1.00 + 2.60 = 8.13. Your personal inflation on this basket is about 8.1%.
Now check it with the shortcut. The same basket at new prices: 14.40 + 23.10 + 21.60 + 42.00 + 12.00 + 5.70 + 13.50 + 29.90 = $162.20. The change is $162.20 − $150.00 = $12.20, and $12.20 ÷ $150.00 = 8.13%. Same answer.
A few things jump out:
- Two items did most of the damage. Coffee and olive oil are 29% of the basket but add 5.0 of the 8.1 points. That's where switching brands, buying in bulk or shopping around pays off the most.
- Chicken barely moved, but it still matters. A 5% rise sounds mild, yet it's the biggest share of the basket, so it adds a full 1.33 points.
- Eggs and coffee rose by the same 20%, but coffee added 2.40 points and eggs only 1.60, because you spend more on coffee.
- Cheaper items help. Cereal and bananas knocked 1.2 points off the total. Nobody puts those in headlines.
Once you know this, the question changes from "why is everything so expensive?" to "what are the two or three items driving my bill?" That's a question you can do something about. Our guide to saving money on groceries has ideas for exactly those swaps.
Traps to avoid
- Shrinkflation. If a bag of chips went from 10 oz to 9 oz at the same price, the price per ounce rose about 11%. Compare unit prices (per ounce, per pound, per count) whenever package sizes change.
- Switching products. If you moved from name-brand to store-brand cereal, the drop isn't the cereal getting cheaper; it's you changing what you buy. Keep the comparison to the same item, or note the switch separately.
- Sales. One week's promo price can make an item look cheaper or pricier than it really is. Averaging several receipts per period smooths that out.
- Different stores. The same item often costs different amounts at different stores. Compare the same store, or at least average consistently.
Track it monthly
Doing this by hand once is eye-opening. Doing it every month is a chore — you'd need to find matching items across dozens of receipts, handle sale prices and keep the math straight.
That's the part Tratobor does for you. Photograph a receipt or upload a PDF, and each line item and price is read off the image; you review the lines and save. As receipts pile up, Tratobor compares the prices of items you bought in both the earlier and later part of the period, weighs each item by how much of it you actually buy, and shows your inflation as one number, with the items behind it.
Alongside it you get:
- What got pricier and what got cheaper, item by item, so you can see which two or three things are driving your number.
- Where it's cheaper — the same item across the stores you shop at, so if olive oil is $2 less at the other store, you'll see it.
- Repeat purchases, the things you buy most often, which is where a small price change adds up.
It's free. There are two limits: 15 receipt photos a month and analysis over the last three months, so your inflation number covers that window. Records are never deleted. The receipt photo is read on the server and not stored.
If you'd rather start with the big picture first, set a target in our grocery budget guide, then use your personal inflation number to adjust it when prices move. A budget that was right a year ago can be quietly 8% too small today — and now you'll know exactly why.