How to Calculate CAC for Marketplaces: The Two-Sided Formula

Most marketplace founders calculate CAC the same way a SaaS company would. Total marketing spend divided by new customers. Simple.

Except it's wrong.

A marketplace has two customer types: buyers and sellers. Riders and drivers. Guests and hosts. Freelancers and clients. The question "what's your CAC?" has no single answer. And depending on which side you're measuring — or whether you're blending them together — your CAC changes by 2-5x.

I've seen founders walk into board meetings quoting a $15 CAC when their real fully loaded number was closer to $70. They weren't lying. They were just counting only the demand side and ignoring what it cost to acquire and onboard supply. I learned this the hard way at PizzaPortal, the food-delivery marketplace I co-founded — restaurant acquisition costs were a very different animal from the cost of getting a hungry customer to order.

That's a $55 gap that will kill your unit economics.

Start Here: Per-Side CAC

Before anything else, you need two separate numbers.

These are your foundation. Every operational decision depends on knowing them individually.

When Uber spends on rider acquisition (demand side), that budget has nothing to do with driver acquisition (supply side). The channels are different. The costs are different. The payback periods are different. Combining them into one number is like averaging the temperature of a freezer and an oven — technically accurate, practically useless.

What counts as demand-side acquisition spend

What counts as supply-side acquisition spend

The supply side almost always costs more per user. Airbnb spends far more to onboard a new host (photography, listing optimization, trust verification) than to acquire a new guest. DoorDash spends more to recruit and onboard a Dasher than to get a hungry customer to download the app.

This asymmetry is the norm, not the exception.

The Two-Sided CAC Formula

Tracking per-side CAC is necessary but not sufficient. You also need a combined metric that reflects the true cost of generating one transaction-ready unit on your marketplace.

Gianluca Valentini's framework solves this. It's analogous to WACC (weighted average cost of capital) in finance — it weights each side's CAC by how much of that cost is attributable to serving a single buyer.

The logic: if one supplier serves many buyers, the cost of acquiring that supplier gets spread across all the buyers they serve. You don't need to recruit a new driver for every Uber rider. One driver handles dozens of rides per week.

Worked Example

Same marketing spend. Same acquisition costs. Nearly 3x the effective CAC. This is why marketplace operators obsess over supply utilization. It's not just an ops metric — it directly moves your CAC.

As I wrote in The Two Numbers That Build or Break Every Business, the relationship between LTV and CAC determines whether a business model works. In marketplaces, that CAC number is structurally more complex — and founders who don't account for both sides are flying blind on one of the two numbers that matter most.

Fully Loaded CAC: What Actually Goes In

Most founders undercount their CAC by 30-50%. They include ad spend and stop there. A fully loaded marketplace CAC captures every dollar you spend to get a new user from "never heard of you" to "completed their first transaction."

Hard costs (easy to track)

Soft costs (often missed)

Quasi-variable costs (almost always missed)

These are the sneaky ones. Fraud processing doesn't scale linearly, but it doesn't stay flat either. Customer support reps spend a disproportionate amount of time on new users versus retained ones. Returns and chargebacks spike during a user's first few transactions.

Etsy, for example, invests heavily in new seller education — webinars, listing optimization tools, shipping label integration, and payment setup assistance. None of that shows up in "ad spend," but it's a real cost of supply-side acquisition.

Upwork spends on freelancer vetting, skill testing, and portfolio review. Fiverr invests in seller onboarding flows and category matching. These costs are part of CAC even though no one in marketing authorized them.

If you're not sure whether a cost belongs in CAC, ask: "Would this cost go away if we stopped acquiring new users?" If yes, it's acquisition cost.

Organic Loops and Viral Effects

Paid acquisition is only part of the story. The best marketplaces build organic acquisition loops that reduce effective CAC over time.

The K-Factor

The viral coefficient measures how many new users each existing user brings in:

A K-factor of 1.0 means each user replaces themselves. Above 1.0 and you get exponential growth without additional spend. Below 1.0 (which is most companies), viral effects still reduce your effective CAC.

The Dropbox Lesson

Dropbox's referral program generated 60% of all new signups at its peak. Their paid CAC was around $300-400 per customer (they briefly tried Google Ads at $233-388 per acquisition against a $99 product — obviously unsustainable). The referral program brought their effective CAC down by more than 50%, making the unit economics work.

Marketplaces can build similar loops. Airbnb hosts share their listings on social media, driving demand-side traffic at zero acquisition cost to Airbnb. Uber rider referral codes became a cultural phenomenon in 2014-2016. Etsy sellers promote their own shops on Instagram, Pinterest, and TikTok — each post is free demand acquisition for the platform.

Supply-Driven Demand: The Goldilocks Zone

Casey Winters (former growth lead at Pinterest and Grubhub) identified a pattern: healthy marketplaces get 10-40% of their demand-side traffic from supply-side activity.

Below 10%, your supply side isn't contributing to demand generation, and you're paying full price for every buyer. Above 40%, you're over-reliant on supply-side-generated demand, which means you lose control of your growth rate.

Etsy sits comfortably in this range — sellers actively market their shops, but Etsy also drives independent demand through SEO, paid acquisition, and brand marketing.

DoorDash, by contrast, generates almost no demand from restaurant activity. Restaurants don't promote their DoorDash listings — if anything, they'd prefer customers order directly. DoorDash has to pay for virtually all demand-side acquisition, which is one reason their CAC runs higher than marketplace averages.

Benchmarks: What Good Looks Like

Every marketplace is different, but investors at Series A have consistent expectations:

B2B vs B2C Differences

B2B marketplaces (Upwork, Toptal, Faire) run structurally higher CAC on both sides. A single enterprise client on Upwork might cost $500-2,000 to acquire through outbound sales. But that client might spend $50,000-200,000+ per year on the platform. The LTV:CAC ratio can be extraordinary even with high absolute CAC.

B2C marketplaces (Uber, Airbnb, Etsy) live and die on volume. CAC must be low in absolute terms because individual LTV is lower. A rider taking 2-3 Uber trips per month at $15-25 per trip generates maybe $30-50 in gross revenue per month for Uber. If rider CAC exceeds $30-40, the payback math gets tight.

Service vs Product Marketplaces

Service marketplaces (TaskRabbit, Fiverr, Thumbtack) tend to have higher supply-side CAC because supply requires vetting, training, and sometimes certification. Product marketplaces (Etsy, Poshmark, StockX) have lower supply-side CAC because listing a product is self-serve.

But service marketplaces often have better supply-side retention. A plumber on Thumbtack who builds a client base through the platform has high switching costs. An Etsy seller can simultaneously list on Amazon Handmade, Shopify, and their own website. The interplay between acquisition cost and retention — between CAC and LTV — is what determines whether the model works.

The 5 Most Common Marketplace CAC Mistakes

Lech Kaniuk covers these pitfalls in detail in The Two Numbers That Build or Break Every Business, including how to model CAC deterioration and identify the inflection point where your CAC trend breaks your LTV:CAC ratio.

Frequently Asked Questions

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