There are three types of fields that involve numbers: Number Fields, Calculation Fields, and Summary Fields. For each of those field types, you can set how many decimal places that field will accept when users are entering data on their mobile devices.
By default, fields will be set to not allow for any decimal places.
Note: Number Fields set to Currency Style (or calculations and summaries that include those fields) will always allow for two (and only two) decimal places. You are not able to edit this setting for those fields.
Configuring fields to allow for decimal places
To set the decimal place, click on the field you would like to configure, and then select the More link in your field settings. You'll see a Decimal Places dropdown:
Set the Decimal Places dropdown to the desired number and then save and publish your form as usual.
If the Decimal Places dropdown is grayed out and unable to be edited, your field is either set to Currency Style or, in the case of Summary and Calculation Fields, includes a field that is set to Currency Style.
Calculation Fields based on multiple Number Fields
Calculation Fields will default to the highest number of decimal places allowed by a field being used in the calculation. If you change the decimal places dropdown to be lower than the highest number, the field will round down to that number of places.
In the below screenshot, both Calculation Fields are set to multiply the "Number 1" field by the "Number 2" field. The "Calculation default" field is set to the default number of decimal places, which is 4 digits (based on the Number 2 field above). The "Calculation no decimal places" was adjusted to allow for no decimal places, so the calculation is being rounded to the nearest whole number: