Studio & Tools · Utilities · on the bench
Paper Tape Calculator
Eleven calculators on one exportable tape, decimal-clean in our run. Its currency rates are examples, not live.
Scorecard · 18 of 20 checks met
How the checks workMissed S2: the currency rates are built-in examples, not read from a reference source.
All four: one label, 'Example rates', serves as both the stated limit and the flag.
All four: the example rate was checked against the ECB and Xe and differed as labelled.
All four: the calculator's own code fetches nothing at all.
Missed R3: eleven calculators against about 200.
01What it is
The Paper Tape Calculator puts eleven calculators behind one adding-machine tape: standard, scientific, programmer, units, currency, loan and mortgage, percentage and tip, dates, age, BMI and health, and statistics. Every result prints to the tape, where it can be reused, explained, printed, or exported as text, Markdown, CSV, JSON, PNG or PDF.
It is for people who calculate in sequences and want a record (bookkeepers, students, anyone checking a loan or a bill) and for keyboard users.
02How it works
All in the browser, from one 873,685-byte script assembled from modules (calculator/assets/js/calculator.js). The tape keeps up to 2,000 lines in the browser's localStorage between visits (calculator.js:424-433, 4565-4570). Decimal answers are cleaned to fifteen significant digits so binary noise never shows (calculator.js:320-348). The exporters write text, a Markdown table, CSV to RFC 4180 with a byte-order mark, and JSON that can be imported again (calculator.js:5897-5912), plus PNG and PDF.
The currency mode lists thirty-one currencies with example rates the reader edits, labelled 'Example rates — edit to match today’s'; it fetches nothing (calculator.js:13122-13160). The whole calculator is built to load in JavaScriptCore with no page at all, so its libraries can be tested on their own (calculator.js:1-30).
03The test
On 27 September 2026 we loaded calculator.js headless in JavaScriptCore, called its libraries directly, and set its built-in euro and yen rates beside the ECB reference rate and Xe's converter.
| UTC | What we did | What came back |
|---|---|---|
| 20:00:19Z | Page and script on the local copy | HTTP 200, 45,265 bytes; calculator.js 873,685 bytes. |
| 20:01:16Z | Libraries, headless | Eleven present, from standard to stats. |
| 20:01:16Z | exact.add(0.1, 0.2) and exact.mul(1.1, 1.1) | 0.3 and 1.21; plain JavaScript gives 0.30000000000000004 and 1.2100000000000002. |
| 20:01:16Z | loan.payment: $300,000 at 6% over 360 months | $1,798.65 a month. |
| 20:01:16Z | stats on 2, 4, 4, 4, 5, 5, 7, 9 | Mean 5; sample standard deviation 2.1381. |
| 20:02:38Z | Built-in EUR rate against the market | Example 0.92 per dollar; ECB reference 0.87696 (25 Sep); Xe 0.87798 at 20:05 UTC, about 5% apart. Yen: example 150, ECB 157.59. |
04The comparison
Calculator.net
fetched 2026-09-27What it is
A free collection of around 200 calculators across finance, fitness and health, maths and other topics, with no registration.
What it does better
Around 200 specialised calculators against eleven, each a focused page for one question.
Where Paper Tape Calculator goes further
One tape runs across every mode, survives between visits up to 2,000 lines and exports six ways (calculator.js:4565-4570, 5897-5912); the Calculator.net page as fetched mentions neither a history nor an export. Its decimal answers are clean, as measured above.
Xe Currency Converter
fetched 2026-09-27What it is
A live converter quoting mid-market rates for 100+ currencies, with alerts, charts and money transfer.
What it does better
Live mid-market rates, 0.87798 euros to the dollar at 20:05 UTC on the day, with rate alerts and history. The labs converter has example rates only.
Where Paper Tape Calculator goes further
Each conversion prints to the tape with its working, alongside ten other calculators, and exports with everything else (calculator.js:13128-13130). Its currency code fetches nothing (calculator.js:13122-13145), so a conversion never waits on a rate service or sends your figures to one.
05Where it falls short
- Currency is not live. The thirty-one currencies carry fixed example rates you must edit; on the day, the euro example was about 5% off the ECB reference. The label says so, but a hurried reader may not look.
- Eleven calculators against Calculator.net's roughly 200: nothing for retirement, pregnancy or concrete volumes.
- Double-precision arithmetic cleaned to fifteen significant digits; it is not an arbitrary-precision calculator.
- A large single script, 873,685 bytes, to load for a calculator.
06The ruling
The Paper Tape Calculator goes to lengths over the record rather than the sum. Every result from every mode lands on one tape that outlives the visit and leaves in six formats, and the arithmetic behind it held up headless: 0.3 where plain JavaScript says otherwise, and a 30-year loan payment of $1,798.65.
Its currency mode is honest about being an example, and our check showed why that honesty matters: the euro figure was five per cent from the market. Use it when you want the working kept; use Xe when you need today's rate, then type it in.
On the docket: 18 of 20 checks met. Eleven calculators on one exportable tape, decimal-clean in our run; its currency rates are examples, not live.