How CS2 Chooses the Output Skin
When you execute a trade-up contract, CS2 determines the output skin through a two-step process:Step 1: Build the Output Pool
The game looks at the collections represented by your 10 input skins. For each collection, it identifies all skins at the next rarity tier. These skins form the output pool — the set of possible results.
Step 2: Weight by Collection Representation
Each collection's share of the output pool is proportional to how many of your 10 inputs came from that collection. If 7 inputs are from Collection A and 3 from Collection B, then:
- Collection A's outputs have a combined 70% probability
- Collection B's outputs have a combined 30% probability
Within a Collection
If a collection has multiple skins at the output rarity tier, the probability is split equally among them within that collection's share. For example, if Collection A has 2 Classified skins and gets 70% weight, each Classified skin from Collection A has a 35% chance (70% / 2).
This weighting system is the foundation of all trade-up strategy. By choosing how many skins to use from each collection, you directly control the probability of each possible output.
Collection Weighting: The Core Mechanic
Collection weighting is what separates profitable trade-ups from unprofitable ones. Here is how it works in practice:Single-Collection Contract (10/10)
When all 10 inputs come from one collection, 100% of the output probability goes to that collection's output skins. If the collection has 3 Classified skins, each has a 33.3% chance. This is the simplest scenario to analyse.
Two-Collection Split
Common splits and their effects:
- 9/1 split: 90% from main collection, 10% from secondary. Use when you want to heavily favor one collection but need one filler from another.
- 7/3 split: 70/30 weighting. Good for accessing a valuable secondary output while keeping primary odds high.
- 5/5 split: Equal 50/50 weighting. Use when both collections have desirable outputs.
Strategic Collection Mixing
The key insight: you can dilute bad outputs by mixing in a collection with only good outputs, or concentrate probability on expensive skins by maximizing that collection's representation. TradeUpX calculates the exact probability and EV for every possible collection split automatically.
For example, if Collection A has one €80 output and one €5 output (each 50% in a 10/10 contract), but Collection B has only one €60 output, a 5A/5B split gives you: 25% chance of €80, 25% chance of €5, 50% chance of €60. The EV shifts dramatically compared to 10/10 from Collection A.
Probability Tables: Understanding Your Odds
Here is how to read and calculate trade-up probabilities for any contract:Formula
P(output_skin) = (inputs_from_collection / 10) × (1 / skins_at_output_tier_in_collection)Example: 7 from Collection A + 3 from Collection B
Collection A has 2 Classified skins (Skin X, Skin Y). Collection B has 1 Classified skin (Skin Z).
- P(Skin X) = (7/10) × (1/2) = 35%
- P(Skin Y) = (7/10) × (1/2) = 35%
- P(Skin Z) = (3/10) × (1/1) = 30%
Total: 35% + 35% + 30% = 100%. Every possible output has a known probability.
Why This Matters for EV
Expected Value is simply each probability multiplied by the corresponding output price (after Steam fees), summed together, minus your input cost:
EV = Σ(P_i × Price_i × 0.87) - Input_CostWhen you change the collection split — say from 7/3 to 8/2 — every probability shifts, and so does the EV. TradeUpX evaluates all possible splits to find the one that maximizes your expected return.
How Mixing Collections Changes Your Odds
Mixing collections is one of the most powerful (and most misunderstood) tactics in CS2 trade-ups. Here are the key scenarios:Scenario 1: Diluting a Bad Output
Collection A has 2 outputs: one worth €100 and one worth €2. In a 10/10 contract, you have a 50% chance of each. By mixing in Collection B (which has one output worth €40), you can reduce the chance of the €2 output. A 5A/5B split gives: 25% × €100, 25% × €2, 50% × €40 — much better EV.
Scenario 2: Accessing an Expensive Exclusive Output
Some collections have a single extremely valuable output at the next tier. Even adding just 1 or 2 inputs from that collection gives you a 10–20% chance at the premium output. If the collection's inputs are expensive, mixing lets you access the output pool without committing all 10 slots.
Scenario 3: When NOT to Mix
If your primary collection has only desirable outputs at the next tier, mixing introduces unnecessary risk from the secondary collection's potentially worse outputs. Always check what the secondary collection contributes to the output pool before mixing.
TradeUpX shows the complete output pool with probabilities and prices for every combination it evaluates. Use the output breakdown to verify that your collection mix improves rather than hurts the contract's expected value.
Advanced: Three or More Collections in One Contract
While most profitable trade-ups use 1–2 collections, you can theoretically use up to 10 different collections (one per input skin). Each collection's output weight equals its share of the 10 inputs.When Multi-Collection Contracts Work
Occasionally, the cheapest inputs across an entire rarity tier come from many different collections. If each collection contributes desirable outputs, a multi-collection contract can work — but the probability math gets complex. With 4 collections having 2 outputs each, you have 8 possible results with varying probabilities.
The Complexity Trade-Off
More collections means more possible outputs, which typically means lower probability per individual output. High-value jackpot outcomes become less likely. This increases variance unless all outputs are similarly priced.
Practical Advice
Stick to 1–2 collections for most contracts. Use 3+ collections only when TradeUpX shows a clear EV advantage and you understand the full output distribution. The scanner evaluates multi-collection combinations automatically, so you do not need to calculate these by hand.
For every result card in TradeUpX, click to expand the full output probability table. This shows you every possible output skin, its probability, its current market price, and its contribution to the contract's total EV.