Imperial Stone Feet

This application is an Imperial units and US customary units Calculator. (I call merely 'Imperial units' below.)
For example, this can calculate "2feet5inch ＋ 6feet9inch ＝ 9feet2inch". Conversion is very common, but the direct calculation keeping units is rare, I think.
The possible calculation unit types are limited to Length/Area/Volume/Mass.
It includes following functions these other unit converters normally do not have.
* Fraction input and result. For example, 5_1/4 inch.
* All types of metric (SI) prefix like kilo, can be used.

It equips User Interface suitable for Touchscreen. (Direct touch to displayed number works as number input or modification, or switching between result and input.)

(Spec)
・Addition (4 items) 2 sheets, Multiplication (4 items) 2 sheets, and simple total (2 items) 1 sheet.
・Addition sheet calculation is switchable from 3 kinds (including proportion). Multiplication sheet calculation is switchable from 2 kinds.
・Every number field of length/area/volume/mass is switchable and convertible from 3 kind formats. (Plural Imperial units connected/Single Imperial unit/Metric unit)
・Number input and modification by direct touch action to the digit you want to change. In re-calculation of slight changes, entering full number is not necessary.
・Number input in keyboard sheet with keys that according to the number type to input.
・The direction of calculation is switchable by touch. Inverse calculation, changing result field to input, is easy.
・Calculation result is automatically done at the time necessary inputs reach.
・At the pressed moment of the key, the key function is shown display. (Not the released moment.)
・Lock of each number is possible.
・Internal calculation: 16-digit decimal or fraction, independently developed (Not binary floating/double)
・Display number length is selected from 9/13-digit depending on screen size. (This digit becomes fewer when the number includes units.)
・2 sheets can be parallelly shown and operated if screen size is larger.
・Copy / paste function of a each number and a sheet.
・2 memories
・Selectable rounding position(6), denominator in fraction(5) and rounding mode(3).
・Plural Imperial units connection provides choices of units to use. And Imperial unit provides choices of differences like UK and US.
・Metric units are switchable from using metric (SI) prefix of -2,-1, 1, 2 exponent or not, or exponential number.
・Automatic save of the calculation state at the application termination.
・Whole initializing. Or returning to the state at the time application started.
・Dark theme(Android10)