Formatting Numbers of Unknown Order of Magnitude

Sometimes, I write programs that need to display numbers where I don't know how big or small they will be. The most common approaches of printing numbers won't be very helpful in such a case.

# Default print: more decimal places than useful
>>> print(1324325425.3254435363463)
1324325425.3254435

# Fixed amount of decimal places: better, but ...
>>> print("{:.0f}".format(1324325425.3254435363463))
1324325425
# ...useless results for small numbers
>>> print("{:.0f}".format(0.3254435363463))
0

A good way to deal with this problem is to use the scientific notation, which displays the mantissa and exponent separately.

>>> print("{:e}".format(1324325425.3254435363463))
1.324325e+09
>>> print("{:e}".format(0.3254435363463))
3.254435e-01

However, not everyone is used to reading the scientific notation and many programs don't accept it as input. What I often want to have is a format that:

• does not irritate humans
• is understood by all programs
• is not unnecessarily verbose

One way to reach this is to round to a fixed number of significant figures.

>>> def fmt_sig(x, sig_figures=3):
...     show_dec = -floor(log10(abs(x)) + 1) + sig_figures
...     return round(x, show_dec)
...
>>> print(fmt_sig(1324325425.3254435363463))
1320000000.0
>>> print(fmt_sig(0.3254435363463))
0.325

But rounding the big number can be confusing because it only has an accuracy of three digits while showing eleven digits. My suggested solution is to format numbers with a minimum number of significant figures and to print all places before the decimal point for big numbers.

>>> def fmt_min_sig(x, min_sig_figures=3):
...     show_dec = max(-floor(log10(abs(x)) + 1) + min_sig_figures, 0)
...     return ("{:." + str(show_dec) + "f}").format(x)
... 
>>> print(fmt_min_sig(1324325425.3254435363463))
1324325425
>>> print(fmt_min_sig(0.3254435363463))
0.325
>>> print(fmt_min_sig(12.3254435363463))
12.3

This approach has worked well for me, but I have not seen it any other code bases. That makes me wonder: Did I miss any downsides? Are others doing the same and I just didn't notice? Or is there a better way to do the same? If you have the answer to one of these questions, please let me know!

Update (2026)

It turns out that JavaScript's Intl.NumberFormat supports this formatting strategy since the V3 proposal shipped in browsers. The ICU library that powers it calls the underlying concept "relaxed precision". You can get the same behavior as fmt_min_sig with:

const fmt = new Intl.NumberFormat("en", {
  minimumSignificantDigits: 3,
  maximumSignificantDigits: 3,
  maximumFractionDigits: 0,
  roundingPriority: "morePrecision"
});
fmt.format(0.0123456)  // '0.0123'
fmt.format(123456.78)  // '123,457'

The morePrecision mode resolves conflicts between the two rounding strategies by picking whichever produces more digits. For large numbers, maximumFractionDigits: 0 wins (keeping all integer digits). For small numbers, minimumSignificantDigits: 3 wins (keeping three significant figures). That is exactly the max() in fmt_min_sig.