Calculate increase per day from two values and a date range or duration, days needed to reach a target, and the projected value after any number of days.
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Increase Per Day Formula
Choose daily change from dates or duration, days to a target using a fixed daily amount or compound daily percentage, or a future value using either model. These are constant-rate mathematical scenarios, not forecasts. Optional unit labels change text only. Maximum decimal places affect display; small nonzero answers use scientific notation.
From dates and known duration both compute the average daily change:
Reach target solves for the days needed at a fixed daily change:
The simple daily percent and the compound daily rate are also reported:
- Initial: starting value at the start date or day 0.
- Final: ending value at the end date or after the duration.
- Days: elapsed time in days. Weeks, months, and years are converted using 7, 30.4375, and 365.25.
- Target: value you want to reach.
- Current: value you are starting from.
- Daily increase: the per-day change you plan to apply.
The date-range task counts elapsed calendar days. The duration task accepts days, weeks, average months or average years. Target tasks show unrounded time and first whole-day crossing, plus an optional target date. Future-value tasks show constant-rate milestones. For compound targets, time = [ln(target) − ln(initial)] ÷ ln(1 + rate). Positive values and a daily percentage greater than−100% are required. A target equal to the start is already reached at day0, including at a zero rate.
Reference Tables
Use these to sanity check a result or convert between rates.
| Time span | Days used | Multiply daily by |
|---|---|---|
| 1 week | 7 | 7 |
| 1 month | 30.4375 | 30.44 |
| 1 quarter | 91.31 | 91.31 |
| 1 year | 365.25 | 365.25 |
| Initial | Final | Days | Per day | Simple % / day |
|---|---|---|---|---|
| 100 | 150 | 10 | +5 | +5% |
| 500 | 650 | 30 | +5 | +1% |
| 1,000 | 1,200 | 365 | +0.548 | +0.0548% |
| 200 | 170 | 15 | -2 | -1% |
Examples and FAQ
Example 1. A subscriber count goes from 4,200 on March 1 to 5,040 on March 31. That is 30 days. Increase per day = (5,040 – 4,200) / 30 = 28 subscribers per day. Simple percent per day = 28 / 4,200 = 0.667% per day.
Example 2. You weigh 180 lb and want to reach 200 lb at 0.25 lb per day. Days = (200 – 180) / 0.25 = 80 days. If you enter a start date, the projected target date is 80 days later. This arithmetic example is not a recommended weight-change rate.
Should I use simple or compound daily rate? Use simple when the change is added in fixed amounts, like a steady daily gain. Use compound when the value grows on top of itself, like interest or population. The compound rate is only defined when both initial and final values are positive.
Why does the calculator use 30.4375 days for a month? That is the average month length over a 4-year cycle (365.25 / 12). It avoids bias from picking a 30- or 31-day month.
Can the increase be negative? Yes. If the final value is below the initial, the per-day result is negative and the status shows “Decreasing.” In the fixed-amount target task, a negative daily change is valid only if your target is below the current value.
Does the date range include both endpoints? No. The day count is the difference between the two dates, so March 1 to March 31 is 30 days, not 31. If your problem counts both endpoints, add one day to the end date.
