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Dallas McLaughlin Digital Marketing

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Percent Change Calculator

The Percent Change Calculator takes a starting value and ending value and returns the percentage increase or decrease — the fastest way to report week-over-week or month-over-month performance shifts accurately.

Enter your starting (original) value and your ending (new) value to calculate the percentage change between the two periods.

About Percent Change Calculator

Percent change is one of the most frequently used calculations in paid media reporting. It answers the question every performance review starts with: how did this metric change compared to the previous period?

The formula: ((New Value − Original Value) ÷ Original Value) × 100 = Percent Change. A metric that moved from 400 to 520 increased by 30%. A metric that moved from 400 to 300 decreased by 25%. The sign — positive or negative — tells the direction; the magnitude tells the degree of change.

Media buyers apply percent change across every reportable metric: CPC, CTR, CPA, conversion rate, impression volume, spend, revenue, and ROAS. Reporting absolute numbers without context gives decision-makers an incomplete picture. Saying CPA was $48 last month tells you a cost; saying CPA decreased 18% month-over-month tells you whether the campaign is moving in the right direction.

Percent change is also essential for flagging anomalies. A 60% spike in CPM week-over-week may indicate an auction dynamic shift, seasonal competition increase, or a targeting issue worth investigating. A 40% drop in CTR may signal creative fatigue or an audience frequency problem. Percent change creates the contrast that makes patterns visible.

One common error: comparing percent changes across periods of different lengths (e.g., a 28-day month versus a 31-day month) without adjusting for days. Always verify that comparisons are apples-to-apples before presenting performance changes to stakeholders.

Use this tool alongside the CPA Calculator and ROAS Calculator when building period-over-period performance reports.

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Paid Media Metrics: Key Terms, Formulas, and Definitions

Spend
Spend is the net advertising cost - the hard cost of media only. This does not include management fees, creative production, or any other service fees.
Impressions
Impressions are the number of times an ad was shown in a given channel - the total volume of ad views delivered by the advertising spend.
Cost-per-thousand Impressions (CPM)
CPM is the cost of purchasing 1,000 impressions. It is the standard buying metric for display, programmatic, and streaming TV advertising. Formula: (Spend ÷ Impressions) × 1,000 = CPM
Click-through Rate (CTR)
CTR is the rate at which impressions result in a click. If 1,000 ads are shown and 10 people click, the CTR is 1%. Formula: Clicks ÷ Impressions = CTR
Clicks
Clicks are the total number of times an ad was clicked. The click metric is also commonly used as a proxy for website sessions driven by paid advertising.
Cost-per-click (CPC)
CPC is the average advertising cost paid per click. Formula: Spend ÷ Clicks = CPC
Conversions
Conversions are the total number of desired actions - sales, leads, form submissions, downloads, or other defined goals - attributed to the advertising spend.
Conversion Rate
Conversion rate is the percentage of clicks that result in a conversion action - a lead, purchase, download, or other defined goal. Formula: Conversions ÷ Clicks = Conversion Rate
Cost-per-acquisition (CPA)
Cost-per-acquisition (CPA), also called cost-per-conversion, is the average amount of advertising spend required to generate one conversion action. Formula: Spend ÷ Conversions = CPA
Return on Ad Spend (ROAS)
ROAS measures how much revenue is generated per dollar of advertising spend. A 4x ROAS means $4 in revenue for every $1 spent - but profitability depends on your margin. Formula: Revenue ÷ Spend = ROAS
Markup
Markup is the percentage added above the cost of a product to arrive at the selling price. It is calculated relative to cost, not revenue - which is the key distinction between markup and margin. Formula: (Selling Price − Cost) ÷ Cost = Markup %
Margin
Gross margin is the percentage of revenue retained after deducting the cost of goods sold. Unlike markup (which is based on cost), margin is calculated relative to revenue - the number that ties directly to advertising profitability targets. Formula: (Revenue − Cost) ÷ Revenue = Margin %
Revenue
Revenue is the total income generated from sales before any costs are deducted. In paid media analysis, revenue attributed to advertising spend is the numerator in ROAS calculations.
Budget
Budget is the total advertising spend allocated to a campaign or channel for a defined period. Budget planning typically begins with a target CPA or ROAS and works backward from desired conversion volume.

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