Your data
Paste a table straight from Excel or Google Sheets, or drop a CSV file. Nothing is uploaded anywhere — the whole calculation runs inside this browser tab.
CSV, TSV or plain text. Files saved in windows-1251 are detected and re-read automatically, so Cyrillic headers do not turn into garbage.
Parsing options
Columns
Result
Save and share
The drawing library is downloaded only when you press the button, and only on this page.
This page has no server side at all. The table you paste is parsed by JavaScript inside your own browser, the chart is drawn on a canvas element on your machine, and the export file is assembled locally. Nothing is uploaded, stored or written to any log. You can disconnect from the network after the page has loaded and everything will still work — which is the simplest way to verify the claim yourself.
Facts and limits of this method
| Levels computed | 90%, 95% and 99%, all at once |
|---|---|
| Distribution | Student's t with n−1 degrees of freedom, not the normal |
| Critical value | Found by inverting the exact t-distribution, not taken from a table |
| Formula | mean ± t · standard deviation / √n |
| Width behaviour | Shrinks as √n — quadrupling the sample halves the interval |
| Minimum rows | 2 |
When it misleads you
- The interval describes the population mean, not the individual values. A 95% interval of 3050 to 3210 does not mean 95% of baskets fall in that range — most of them fall well outside it. Confusing the two is the single most common misreading of this number.
- It assumes your rows are a random sample of the population you want to describe. Data from one shop, one week or one channel gives a precise interval around the wrong quantity, and no arithmetic can detect that.
- "95% confident" is a statement about the procedure, not about your particular interval. It means that if you repeated the sampling many times, about 95 of every 100 intervals built this way would contain the true mean; yours either does or does not.
- On heavily skewed data the interval is still computed but becomes unreliable, because the t-distribution assumes the sample mean is approximately normally distributed. That holds well at 30+ rows and poorly at 8.
How it is calculated
The half-width is the standard error of the mean multiplied by a critical value from the t-distribution. The standard error is the standard deviation divided by the square root of the count, which is why the interval narrows as data accumulates while the standard deviation itself does not.
The t-distribution is used rather than the normal, and this matters exactly where it is most often ignored. At 10 rows the 95% critical value is 2.26 instead of 1.96 — an interval 15% wider — and that gap is the correction for having estimated the standard deviation from the same small sample.
The critical value is obtained by numerically inverting the exact t-distribution rather than by looking it up in a rounded table. That means the result is correct for any number of rows, including the awkward sizes that printed tables skip.
All three conventional levels are shown together because the choice between them is arbitrary and showing only one hides that. A result that is significant at 95% and not at 99% is a result whose fragility deserves to be visible.
Questions and answers
Does a 95% interval contain 95% of my values?
No, and this is the classic error. It is a range for the population mean. The range covering most individual values is roughly mean ± 2 standard deviations, which is far wider.
Why does my interval shrink so slowly as I add data?
Because it narrows with the square root of the sample size. To halve the width you need four times the rows, and to halve it again you need sixteen times.
Why the t-distribution instead of the normal?
Because the standard deviation was estimated from the same sample, which adds uncertainty. The t-distribution accounts for it. Above about 30 rows the two nearly coincide, but below that the normal gives intervals that are too narrow.
Which level should I report?
95% is the convention in most fields. Report the level you chose alongside the interval — an interval without its confidence level is meaningless.
Is my data uploaded?
No. The critical values and the interval are computed in your browser.