fmt_number_si: Format numbers with SI prefixes

In gt: Easily Create Presentation-Ready Display Tables

Format numbers with SI prefixes

Description

Format numeric values with SI (International System of Units) prefixes, automatically selecting the appropriate prefix to keep the mantissa in a readable range. SI prefixes range from quetta (Q, 10^30) to quecto (q, 10^-30) and are commonly used in scientific and engineering contexts to represent very large or very small quantities with units (e.g., "5.2 kW", "3.8 ng", "1.2 GHz", etc.).

This function provides fine control over SI prefix formatting with the following options:

• unit specification: define a fixed unit or use per-row units from a column

• prefix selection: choose between all SI prefixes or only engineering prefixes (powers of 1000)

• precision control: specify decimal places or significant figures

• spacing: customize the separator between number, prefix, and unit

• locale-based formatting: use locale-specific decimal and thousands separators

Usage

fmt_number_si(
  data,
  columns = everything(),
  rows = everything(),
  unit = NULL,
  prefix_mode = c("engineering", "decimal"),
  decimals = 2,
  n_sigfig = NULL,
  drop_trailing_zeros = FALSE,
  drop_trailing_dec_mark = TRUE,
  use_seps = TRUE,
  scale_by = 1,
  pattern = "{x}",
  sep_mark = ",",
  dec_mark = ".",
  force_sign = FALSE,
  incl_space = TRUE,
  locale = NULL
)

Arguments

data	The gt table data object ⁠obj:<gt_tbl>⁠ // required This is the gt table object that is commonly created through use of the gt() function.
columns	Columns to target <column-targeting expression> // default: everything() Can either be a series of column names provided in c(), a vector of column indices, or a select helper function (e.g. starts_with(), ends_with(), contains(), matches(), num_range() and everything()).
rows	Rows to target <row-targeting expression> // default: everything() In conjunction with columns, we can specify which of their rows should undergo formatting. The default everything() results in all rows in columns being formatted. Alternatively, we can supply a vector of row captions within c(), a vector of row indices, or a select helper function (e.g. starts_with(), ends_with(), contains(), matches(), num_range(), and everything()). We can also use expressions to filter down to the rows we need (e.g., ⁠[colname_1] > 100 & [colname_2] < 50⁠).
unit	Unit to append to formatted values ⁠scalar<character>⁠ // default: NULL (optional) A character string specifying the unit to append after the SI prefix (e.g., "g" for grams, "W" for watts, "Hz" for hertz, "m" for meters). If NULL, only the prefix will be shown. The unit can also be dynamically specified per row using from_column().
prefix_mode	Type of SI prefixes to use ⁠singl-kw:[engineering|decimal]⁠ // default: "engineering" The type of SI prefixes to use. Options are "engineering" (powers of 1000 only) or "decimal" (all SI prefixes including powers of 10 and 100). See the SI Prefix Modes section for details.
decimals	Number of decimal places scalar<numeric|integer>(val>=0) // default: 2 The exact number of decimal places to display in the mantissa. If both decimals and n_sigfig are provided, n_sigfig takes precedence.
n_sigfig	Number of significant figures scalar<numeric|integer>(val>=1) // default: NULL (optional) Format numbers to n significant figures. This is often preferred in scientific contexts to maintain consistent precision across different magnitudes. When specified, the decimals argument is ignored.
drop_trailing_zeros	Drop trailing zeros ⁠scalar<logical>⁠ // default: FALSE Remove trailing zeros after the decimal point (e.g., "1.50" becomes "1.5").
drop_trailing_dec_mark	Drop trailing decimal mark ⁠scalar<logical>⁠ // default: TRUE Remove the decimal mark if all decimal places are zero (e.g., "1." becomes "1").
use_seps	Use digit group separators ⁠scalar<logical>⁠ // default: TRUE Enable or disable the use of digit separators (e.g., thousands separators).
scale_by	Scale values by a fixed multiplier ⁠scalar<numeric|integer>⁠ // default: 1 All numeric values will be multiplied by the scale_by value before undergoing formatting. Since the default value is 1, no values will be changed unless a different multiplier value is supplied. This is useful for unit conversions, such as using unit_conversion() to convert horsepower to watts before formatting with SI prefixes.
pattern	Decoration pattern ⁠scalar<character>⁠ // default: "{x}" A formatting pattern for decorating values. Use {x} to represent the formatted value (including prefix and unit).
sep_mark	Thousands separator ⁠scalar<character>⁠ // default: "," The character to use as the thousands separator. Overridden if locale is provided.
dec_mark	Decimal mark ⁠scalar<character>⁠ // default: "." The character to use as the decimal point. Overridden if locale is provided.
force_sign	Force positive sign ⁠scalar<logical>⁠ // default: FALSE Force the display of a plus sign for positive values.
incl_space	Include a space between the value and the unit symbol ⁠scalar<logical>⁠ // default: TRUE An option for whether to include a space between the numerical value and the SI prefix + unit (e.g., TRUE for "1.5 kW", FALSE for "1.5kW"). Per SI convention, there should be a space between the value and the unit symbol.
locale	Locale identifier ⁠scalar<character>⁠ // default: NULL (optional) An optional locale identifier for locale-specific number formatting. When provided, overrides sep_mark and dec_mark with locale-appropriate values.

Value

An object of class gt_tbl.

SI Prefix Modes

The prefix_mode argument controls which SI prefixes are used:

• "engineering": Uses only prefixes for powers of 1000. This includes:

  • greater than 1: k (kilo), M (mega), G (giga), T (tera), P (peta), E (exa), Z (zetta), Y (yotta), R (ronna), Q (quetta)

  • less than 1: m (milli), u (micro), n (nano), p (pico), f (femto), a (atto), z (zepto), y (yocto), r (ronto), q (quecto)

  • this is the most common convention in scientific and engineering contexts.

• "decimal": Uses all SI prefixes including those for powers of 10 and 100:

  • Additional prefixes for greater-than-1 values: da (deca), h (hecto)

  • Additional prefixes for less-than-1 values: d (deci), c (centi)

  • This mode is less commonly used but follows the complete SI standard.

Compatibility of formatting function with data values

fmt_number_si() is compatible with body cells that are of the "numeric" or "integer" types. Any other types of body cells are ignored during formatting. This is to say that cells of incompatible data types may be targeted, but there will be no attempt to format them.

Compatibility of arguments with the from_column() helper function

from_column() can be used with certain arguments of fmt_number_si() to obtain varying parameter values from a specified column within the table. This means that each row could be formatted a little bit differently. These arguments provide support for from_column():

• unit: The unit can be specified with a column name in quotes.

• decimals: Each row's number formatting could use a different number of decimal places.

• n_sigfig: Each row could have a different number of significant figures.

• drop_trailing_zeros: The option to drop trailing zeros can be controlled per row.

• drop_trailing_dec_mark: The option to drop trailing decimal marks can be controlled per row.

• use_seps: The use of digit separators can be enabled or disabled on a per-row basis.

• scale_by: The scale multiplier can be different for each row.

• pattern: The formatting pattern can be specified per row.

• sep_mark: The thousands separator mark can be set per row.

• dec_mark: The decimal mark can be set per row.

• force_sign: Whether to force a plus sign can be controlled per row.

• incl_space: Whether to include a space between number and unit can vary per row.

• locale: The locale can be specified per row.

Please note that for all of the aforementioned arguments, a from_column() call needs to reference a column that has data of the correct type (this is different for each argument). Additional columns for parameter values can be generated with cols_add() (if not already present). Columns that contain parameter data can also be hidden from final display with cols_hide(). Finally, there is no limitation to how many arguments the from_column() helper is applied so long as the arguments belong to this closed set.

Adapting output to a specific locale

This formatting function can adapt outputs according to a provided locale value. Examples include "en" for English (United States) and "fr" for French (France). The use of a valid locale ID here means separator and decimal marks will be correct for the given locale. Should any values be provided in sep_mark or dec_mark, they will be overridden by the locale's preferred values.

Note that a locale value provided here will override any global locale setting performed in gt()'s own locale argument (it is settable there as a value received by all other functions that have a locale argument). As a useful reference on which locales are supported, we can call info_locales() to view an info table.

Examples

Create a table showing the masses of obelisks located in Rome. The masses are initially in metric tons, which we'll convert to grams using unit_conversion() in the scale_by argument. The resulting values are then formatted with SI prefixes, which are all here as M (mega).

dplyr::tibble(
  obelisk = c(
    "Lateran Obelisk",
    "Vatican Obelisk",
    "Flaminio Obelisk",
    "Pantheon Obelisk"
  ),
  mass_ton = c(455, 331, 235, 30)
) |>
  gt() |>
  fmt_number_si(
    columns = mass_ton,
    unit = "g",
    decimals = 0,
    scale_by = unit_conversion(
      from = "mass.metric-ton",
      to = "mass.gram"
    )
  ) |>
  cols_label(
    obelisk = "Obelisk",
    mass_ton = "Mass"
  )

Create a table showing measurements of different substances with varying units. The unit column contains different units per row (grams and liters), which are used with from_column() to apply appropriate SI prefixes.

dplyr::tibble(
  substance = c("Glucose", "Vitamin C", "Caffeine", "Water"),
  amount = c(0.0051, 0.000075, 0.0002, 0.250),
  unit = c("g", "g", "g", "L")
) |>
  gt() |>
  fmt_number_si(
    columns = amount,
    unit = from_column("unit"),
    n_sigfig = 2
  ) |>
  cols_hide(columns = unit)

You can combine fmt_number_si() with fmt_units() and cols_merge() to format measurements with SI prefixes on units that need special typesetting. In this example, fmt_number_si() handles both the SI prefix and the unit with proper spacing, while fmt_units() creates a separate column with additional unit information (like "per hour") that gets merged in.

dplyr::tibble(
  measurement = c("Power", "Resistance", "Energy", "Fall Velocity"),
  value = c(1500, 2400000, 3600000,0.033),
  unit = c("W", ":ohm:", "J", "m /s")
) |>
  gt() |>
  fmt_number_si(columns = value, decimals = 1) |>
  fmt_units(columns = unit) |>
  cols_merge(columns = c(value, unit), pattern = "{1}{2}")

Function ID

3-5

Function Introduced

v1.2.0 (December 16, 2025)

See Also

The vector-formatting version of this function: vec_fmt_number_si().

Other data formatting functions: data_color(), fmt(), fmt_auto(), fmt_bins(), fmt_bytes(), fmt_chem(), fmt_country(), fmt_currency(), fmt_date(), fmt_datetime(), fmt_duration(), fmt_email(), fmt_engineering(), fmt_flag(), fmt_fraction(), fmt_icon(), fmt_image(), fmt_index(), fmt_integer(), fmt_markdown(), fmt_number(), fmt_partsper(), fmt_passthrough(), fmt_percent(), fmt_roman(), fmt_scientific(), fmt_spelled_num(), fmt_tf(), fmt_time(), fmt_units(), fmt_url(), sub_large_vals(), sub_missing(), sub_small_vals(), sub_values(), sub_zero()

gt documentation built on Jan. 22, 2026, 9:07 a.m.