Input variables and calculated variables

Input variables and calculated variables are what turn your brew sheet from a checklist into data you can calculate with and report on.

  • An input variable labels a reading or check on a recipe so the same measurement can be tracked across every batch, recipe and recipe version that uses it — your mash temperature, your post-boil gravity, and so on.

  • A calculated variable is a formula that works out a new value from your input variables (and from other calculated variables) — your brewhouse efficiency, your apparent attenuation, the length of your mash.

Both live under Production → View → Recipes → Settings & options, where you'll find an Input variables page and a Calculated variables page.

Breww also ships a set of system input and calculated variables that every account can see. You can't rename or delete these, but you can set your own deviation thresholds on them. If you create your own variable with the same variable name as a system one, yours takes precedence in formulas.

Input variables

Creating an input variable

Go to Settings & options → Input variables → Add new and fill in:

  • Name — what you'll call it, e.g. Post boil gravity.

  • Type — whether this variable is recorded by a Reading action or a Check action on your recipe.

  • Reading type — for readings only, which of the reading types below it is. This determines the units offered, how the value is displayed, and which threshold units apply.

  • Deviation thresholds — optionally, how far a reading can drift from its target before Breww flags it. See Deviation thresholds and targets.

You don't enter the variable name used in formulas — Breww derives it from the name you give. Original gravity becomes original_gravity, Post-boil gravity becomes post_boil_gravity, and a name starting with a digit gets a v_ prefix so it's still valid in a formula (30 HL gravity reading becomes v_30_hl_gravity_reading).

The reading types

Reading type What it records Units
Temperature A temperature °C, °F
Gravity A gravity reading SG, Plato, Brix
pH A pH reading pH
Weight A weight kg, lb
Volume A volume l, hl, gal, bbl
Time of day A clock time, e.g. when you mashed in —
Duration An elapsed length of time, e.g. how long the mash took —
Distance A length or depth mm, cm, m, in, ft
Pressure A pressure bar, psi
CO₂ Dissolved carbon dioxide g/l, mg/l, vol
Percentage Any percentage value — (shown with %)
Dissolved oxygen Dissolved oxygen ppm, ppb, mg/l
Colour Beer colour EBC, SRM
Bitterness Bitterness IBU
Haze Haze/turbidity EBC, NTU, ASBC, FTU
Custom Anything that doesn't fit a standard measurement — recorded as free text —

Because a Custom reading is free text, it can hold anything you like — but Breww can't do maths on it, so it can't be used in a formula or plotted on a comparison chart. If you want to calculate or chart a value, use one of the measured types instead.

Using an input variable on a recipe action

Once created, apply the input variable to the relevant action in your recipe. When you choose Add action → Reading, Breww shows a two-pane picker:

  • The left rail is a searchable list of all the reading types. Start typing to filter it, and each type shows how many of your input variables already use it.

  • The right pane shows the input variables available for the reading type you've selected.

If the reading type you need doesn't yet have a suitable input variable, choose the Other [type] reading option. Breww will offer to create an input variable first (recommended, so the reading is reusable in calculations and reporting), or to continue without one for a genuine one-off.

Recording readings on the brew sheet

On the brew sheet, each reading type gives the brewer the right kind of input — a number plus a unit for measured types, a time picker for Time of day, and a free-text length of time for Duration.

A Duration is typed with its unit, for example 45m or 1h 30m:

You type Breww records
45m 45 minutes
1h 30m 1 hour 30 minutes
90s 1 minute 30 seconds
2d 6h 2 days 6 hours
1w 1 week

The units you can use are seconds (s, sec, secs, second, seconds), minutes (m, min, mins, minute, minutes), hours (h, hour, hours), days (d, day, days) and weeks (w, week, weeks).

Check input variables

An input variable on a Check action records that a yes/no verification step was completed, which is useful for reporting across batches. Checks aren't numeric measurements, so they can't be used inside a calculated variable's formula — only reading input variables can.

Calculated variables

Creating a calculated variable

Go to Settings & options → Calculated variables → Add new and fill in:

  • Name — e.g. Litre degrees. As with input variables, the formula name is derived automatically (litre_degrees).

  • Formula — the calculation itself, written using your variable names.

  • Measurement type — the kind of value the formula produces. This controls how the result is formatted and which units its thresholds use.

  • Deviation thresholds — optionally, the acceptable and warning bands for the result.

Names have to be unique across both input variables and calculated variables (including the system ones), and can't be a reserved programming word. Breww will tell you if the name you've chosen clashes.

Writing a formula

Reference any input variable or calculated variable by its variable name, and combine them with the usual operators:

Operator Meaning
+ - * / Add, subtract, multiply, divide
^ or ** Raise to a power
% Remainder
( ) Grouping, to control the order of calculation

For example, a simple "litre degrees" calculation:

post_boil_gravity * volume_in_fv

A few things worth knowing:

  • Calculated variables can reference other calculated variables, so you can build a complicated figure up in readable steps rather than one enormous formula. Breww detects circular references (A depends on B which depends on A) and won't let you save them, since they can't be worked out.

  • A formula must produce a number, a time of day, or a duration. A formula that produces a yes/no answer (e.g. original_gravity > 1.05) can't be saved as a calculated variable.

  • If a variable was recorded more than once on the batch, Breww averages the values so the result is always well-defined.

  • On multi-turn batches, a formula running on a stage that repeats per turn sees that turn's readings plus any readings from stages that don't repeat per turn. A formula running on a stage that doesn't repeat per turn averages any per-turn readings across all the turns.

The formula editor highlights your syntax, autocompletes your variable names as you type, and flags obvious mistakes before you save. The page also lists every input variable and calculated variable you can reference, and a Functions card showing the functions available to you.

Functions

Function Example What it does
duration duration("1h 30m") A fixed length of time you can add to or subtract from a time or another duration

Measurement types

The measurement type tells Breww what kind of value your formula produces:

Temperature, Gravity, pH, Weight, Volume, Time of day, Duration, Distance, Pressure, CO₂, Percentage, Dissolved oxygen, Colour, Bitterness, Haze, or Unitless.

Pick Unitless for a plain number that isn't a physical measurement (a ratio, an efficiency percentage point, a count). Otherwise pick the type that matches the result — it determines how the value is formatted on the brew sheet and which units are offered for its deviation thresholds.

For Time of day and Duration the measurement type isn't just cosmetic — it has to match what the formula actually works out:

  • A formula that produces a clock time must use Time of day.

  • A formula that produces an elapsed length of time must use Duration.

  • A formula that produces a plain number must use neither of those.

If they don't match, Breww won't save the calculated variable, and will tell you which measurement type the formula needs.

Working with times and durations

Clock times and lengths of time aren't interchangeable, so Breww keeps them apart in formulas. Subtracting one clock time from another gives you a length of time; adding a length of time to a clock time gives you another clock time.

Formula What you get
time of day − time of day a duration (how long between the two)
time of day + duration a time of day (later)
time of day − duration a time of day (earlier)
duration + duration a duration
duration − duration a duration
duration × a number a duration
duration ÷ a number a duration

Some combinations have no sensible meaning, and Breww will tell you so when you save:

  • Adding a plain number to a time of day or a duration. A bare 45 doesn't say whether you mean minutes, hours or days — use duration("45m") instead.

  • Multiplying or dividing a time of day. "Half past nine, doubled" isn't a meaningful value.

  • Subtracting a duration from a plain number, or comparing a time of day against a duration.

Time-of-day readings are always converted to your brewery's own timezone before the calculation runs, using the date the reading was taken so daylight saving is handled for you.

Worked examples

Litre degrees, from a gravity reading and a volume reading:

post_boil_gravity * volume_in_fv

Measurement type: Unitless.

How long the mash took, from two time-of-day input variables:

mash_out_time - mash_in_time

Measurement type: Duration. The brew sheet will show something like 1h 25m.

When to expect knockout, from a single time-of-day input variable:

mash_in_time + duration("4h 30m")

Measurement type: Time of day. The brew sheet will show something like 13:15.

Total time across two stages, from two duration readings or calculations:

mash_length + boil_length

Measurement type: Duration.

Targets and deviation thresholds

Both input variables and calculated variables can have deviation thresholds — the acceptable and warning bands around a target — set centrally on the variable, with optional per-action overrides. Targets themselves are set on the individual recipe action. See Deviation thresholds and targets for the full explanation.

How you enter a target depends on the type:

  • Measured types (temperature, gravity, pH, weight, volume, and so on) — a number plus a unit.

  • Percentage — a plain number; Breww adds the %.

  • Time of day — a clock time, chosen with a time picker.

  • Duration — a length of time, typed as e.g. 45m or 1h 30m.

For Time of day and Duration, the threshold bands are offsets in second, minute, hour or day. So a duration target of 1h 30m with an acceptable band of ± 5 minutes is in spec anywhere from 1h 25m to 1h 35m.

The live preview on the recipe action form renders in the same terms as the brew sheet, so you can check a target and its bands read sensibly before saving.

How values are displayed and used

Once a reading is recorded or a calculation resolves, its value is formatted according to its type — a measured value with its unit, a percentage with %, a time of day as a clock time, and a duration as a span like 1h 30m.

  • A time of day that carries over midnight gets a day marker so it isn't misleading: 01:15 +1d means quarter past one the following morning, and 23:45 -1d means quarter to midnight the previous evening.

  • A negative duration — usually a sign two readings were recorded the wrong way round — is shown with a leading minus, e.g. -15m.

You'll see the same formatting used consistently across:

  • The live brew sheet, where calculations update as the brewer enters readings.

  • The PDF brew sheet, including targets and threshold bands.

  • The Alerts tab of the production dashboard, where out-of-spec readings and calculations are surfaced. See Deviation thresholds and targets for how to choose whether a calculation raises alerts.

  • Batch comparison charts, both from a batch's Analysis tab and on a saved batch comparison dashboard. Axis labels and tooltips are shown as clock times or durations where appropriate rather than raw numbers, and target lines and threshold bands line up with them. In the summary table, the Range column (highest minus lowest) is always shown as a duration for time-of-day variables, because the gap between two clock times is a length of time, not a time of day.

  • Approvals. Approval templates (pre-packaging, post-packaging, pre-transfer and post-transfer) can include a Calculated variable line that pulls the result straight from the batch. When the linked variable is a time of day or a duration, the Target, Acceptable range from and Acceptable range to fields adapt to match — a time picker or a duration field — and the values display as clock times or durations on the approval itself.

  • Reporting, where an input variable lets you compare the same measurement across batches, recipes and recipe versions.

Importing and exporting variables

Input variables and calculated variables are included when you export a recipe as a Breww recipe file, and can be created on import. The file format documents the exact reading_type and measurement_type values, and how targets and threshold blocks are written for each. See Importing and exporting recipes using a Breww recipe file.

What changed for existing accounts

If you were already using time readings in calculations, a few things were corrected automatically:

  1. Times are now read in your brewery's local time. Previously, a time reading used in a formula was treated as UTC, so any brewery not on UTC saw results shifted by their offset — for example mash_in_time + 45 returning 08:23 instead of 09:23. Breww now converts each reading to your brewery's local time first, using the date the reading was taken so daylight saving is handled correctly.

  2. Formulas that added a bare number of minutes were updated for you. A formula like mash_in_time + 45 was rewritten to mash_in_time + duration("45m") so it keeps working and clearly says what the 45 means.

  3. Measurement types were corrected. Calculated variables whose formula produces a clock time or an elapsed length of time had their measurement type set to Time of day or Duration as appropriate. Any variable marked as a time or duration but actually producing a plain number was reset to Unitless.

  4. Existing brew sheets were recalculated, so historical batches show the corrected results rather than the old ones.

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