Function nper 

pub fn nper<C: Into<f64> + Copy, P: Into<f64> + Copy, F: Into<f64> + Copy>(
    periodic_rate: f64,
    payment: C,
    present_value: P,
    future_value: F,
) -> f64

Returns the number of periods required for an annuity (payments) to reach a future value, at a specified periodic rate. Still in beta.

Related functions:

• To calculate nper while retaining the input values use nper_solution.

The NPER formula is:

Number of Periods = LN( (payment - future_value * periodic_rate) / (payment + present_value * periodic_rate) ) / LN(1 + periodic_rate)

…where LN is the natural log (log10).

Arguments

• periodic_rate - The rate at which the investment grows or shrinks per period, expressed as a floating point number. For instance 0.05 would mean 5% growth. Often appears as r or i in formulas.
• payment - The payment amount per period, also referred to as an annuity or cashflow. In this formula, it must be negative, and future value must be positive. Often payment appears as pmt or C (cashflow) in formulas.
• present_value - The present value, or total value of all payments now. Often appears as pv in formulas.
• future_value - The future value, a cash balance after the last payment is made and interest has accrued in each period. Often appears as fv in formulas.

Panics

The call will fail if payment is greater than or equal to 0, because the formula requires payment to be negative. Additionally, present_value and future_value must be positive, or 0. However, both present_value and future_value cannot be 0, at least one value must be set.

Returns f64 for Number of Periods (NPER). Receive the number of periods required for a present value to equal a future value based on a set of payments at an interest rate. If there is no initial present value, use 0. This is equivalent to the NPER function in Excel / Google Sheets.