Rex says
Money math without the spreadsheet headache. Plug in your numbers and I'll show you exactly where the dollars land.
Try a scenario
Click to load — tweak from there.Inputs
Result
Estimated conversions at new window
162
CAC at new window
$154.70
ROAS at new window
0.58
ROAS at current window
1.80
CAC change moving to new window
209.4%
Psst — share this and help Rex grow
One click, a permanent link with your numbers baked in.
How to use this
- 1Enter campaign spend ($).
- 2Enter current attribution window (days).
- 3Enter conversions reported at current window.
- 4Enter new attribution window to compare (days).
- 5Enter average order value ($).
- 6Enter conversion decay rate (higher = faster front-loading).
- 7Read your estimated conversions at new window on the right — it updates as you type.
- 8Hit Share to keep the scenario or send it to someone.
About this calculator
Ad platforms let you choose attribution windows like 1-day click, 7-day click, or 28-day click-and-view, and the same campaign can look wildly different depending on which one you pick — a channel showing 3.0 ROAS on a 28-day window might show 1.4 ROAS on a 7-day window with no change in actual performance. This calculator estimates how conversions and revenue shift when you move between windows using a decay curve: most conversion credit front-loads in the first few days after click, then tails off with diminishing marginal conversions per additional day. Enter your current window's reported conversions and the assumed share of conversions that happen within the first day versus trickling in later, and it estimates what you'd see on a shorter or longer window, along with the resulting CAC and ROAS shift. This matters most when comparing performance across platforms that default to different windows, or when a platform quietly changes its default and your reported numbers jump without any real change in what happened. Always compare channels on the same window, and treat the CAC/ROAS spread across windows as a rough bound on attribution uncertainty, not a precise number.
Worked example
Using the values the calculator loads with:
Inputs
- Campaign spend: 25000 $
- Current attribution window: 7 days
- Conversions reported at current window: 500
- New attribution window to compare: 1 days
- Average order value: 90 $
- Conversion decay rate (higher = faster front-loading): 0.35
Results
- Estimated conversions at new window: 162
- CAC at new window: $154.70
- ROAS at new window: 0.58
- ROAS at current window: 1.8
- CAC change moving to new window: 209.4%
What each field means
Inputs
- Campaign spend ($)
- The campaign spend used in the calculation, measured in $. Starts at 25000 $ so you have a working example on load.
- Current attribution window (days)
- The current attribution window used in the calculation, measured in days. Starts at 7 days so you have a working example on load. Accepted range: 1–90 days.
- Conversions reported at current window
- The conversions reported at current window used in the calculation. Starts at 500 so you have a working example on load.
- New attribution window to compare (days)
- The new attribution window to compare used in the calculation, measured in days. Starts at 1 days so you have a working example on load. Accepted range: 1–90 days.
- Average order value ($)
- The average order value used in the calculation, measured in $. Starts at 90 $ so you have a working example on load.
- Conversion decay rate (higher = faster front-loading)
- The conversion decay rate (higher = faster front-loading) used in the calculation. Starts at 0.35 so you have a working example on load. Accepted range: 0.05–2.
Results
- Estimated conversions at new window
- Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- CAC at new window
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- ROAS at new window
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- ROAS at current window
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- CAC change moving to new window
- Returned as a percentage. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Why does shortening the attribution window always raise reported CAC?
A shorter window credits fewer of the conversions that actually happened, because some people click an ad and buy several days later. Fewer counted conversions for the same spend mechanically raises CAC and lowers ROAS, even though nothing about actual performance changed.
What decay rate should I use?
Impulse, low-consideration purchases (fast fashion, snacks, mobile games) decay fast — try 0.5-1.0. Considered purchases (furniture, software, big-ticket retail) decay slower, try 0.15-0.3, since people research over days or weeks before converting.
How do I pick which window to actually report on?
Pick one window and use it consistently across all channels for internal comparison, ideally matching your typical sales cycle length. When comparing platforms with different default windows, normalize them to the same window using this kind of estimate before drawing conclusions about which channel performs better.
Is this a substitute for a real attribution model?
No — this is a rough directional estimate for understanding reporting artifacts, not a replacement for multi-touch attribution or incrementality testing, which measure causal impact rather than just re-timing the same claimed conversions.
Accuracy and limitations
- Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
- Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.
- This is educational maths, not financial advice. Check anything contractual with the lender or your accountant.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Attribution Window Shift Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/attribution-window-shift-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/attribution-window-shift-calculator" target="_blank" rel="noopener">Attribution Window Shift Calculator — RevenueLab</a> (2026).</p>
Source: [Attribution Window Shift Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/attribution-window-shift-calculator) (2026).