The same rounding place and method apply to both tabs.
Rounded result
Original vs rounded value with error band.
Digit inspected
We highlight both the place you round to and the digit you look at for the rounding decision.
Steps
What you can do
- Round to any decimal or integer place, with the digit you inspected highlighted.
- See the maximum error and the possible true-value band on a number line.
- Compare intermediate vs final rounding on
(A op1 B) op2 Ctemplates. - Copy a shareable URL or LaTeX, download the error-band SVG, and switch to teacher mode.
How to use this calculator effectively
This guide helps you use Rounding & error bands in a repeatable way: define a baseline, change one variable at a time, and explain each output using explicit assumptions before sharing results.
How it works
The calculator applies deterministic formulas to your input values and only rounds at the final display layer. This makes it useful for comparative analysis: keep one scenario as a baseline, then vary assumptions and measure the delta in both absolute terms and percentage terms. If a change appears too large or too small, verify units, period conventions, and sign direction before interpreting the result.
When to use
Use this page when you need a fast planning estimate, a classroom check, or a reproducible scenario that teammates can review. It is most effective at the decision-prep stage, where you need to compare options quickly and decide which assumptions deserve deeper modeling or external validation.
Common mistakes to avoid
- Mixing units such as percent vs decimal, or monthly vs yearly settings.
- Changing multiple fields at once, which hides the real cause of result movement.
- Comparing outputs across tools without aligning constants and default conventions.
- Treating rounded display values as exact inputs for downstream calculations.
Interpretation and worked example
Start with a baseline case and save that output. Next, edit one assumption to reflect your realistic alternative, then compare both the direction and size of change. If the direction matches domain intuition and magnitude is plausible, your setup is likely coherent. If not, check hidden defaults, unit conversions, boundary conditions, and date logic before drawing conclusions.
See also
FAQ
Which digit do I look at when rounding to the nth decimal?
Look at the digit one place to the right of where you round. If it is 5 or more, round up; otherwise round down.
How do I round to the nearest ten?
Use the ones place as the decision digit. For example, 368 becomes 370 (error ±5).
How do I express the error range of an approximation?
With half-up rounding, half of the unit is the maximum absolute error. Rounding to 1 decimal gives ±0.05, so the true value is within that band.
Why does rounding midway change the answer?
Once a rounded value is used in the next operation, its error can accumulate or be amplified, leading to a different final result.
How is rounding of negative numbers handled?
We use “away from zero” half-up. −1.5 becomes −2, while −1.25 becomes −1.
Is my input sent to a server?
No. Everything runs locally in your browser, including the charts.