Oversampling Ratio Calculator

Calculate the target minority-class size after oversampling (e.g. SMOTE) to reach a desired class ratio.

Inputs

Majority Class Countsamples

Minority Class Countsamples

Target Ratio (majority : minority)

e.g. 1 = fully balanced (1:1); 2 = majority is 2x minority after resampling.

New Minority Class Size

4,750

Synthetic Samples Needed

4,250

Current Imbalance Ratio

19.00:1

Step by step

Current ratio
9500 ÷ 500
= 19.00 : 1
New minority class size
9500 ÷ 2
= 4750
Synthetic samples to generate
4750 − 500
= 4250

How it works

Oversampling techniques like SMOTE generate synthetic minority-class samples to shrink the imbalance ratio toward a target value, without touching the majority class. This calculator computes the required new minority class size (majority ÷ target ratio) and the number of synthetic samples needed to reach it — the gap between the new target size and your current minority count.

Formulas

New minority size

new_minority = round(majority_count / target_ratio)

majority_count: Number of majority-class samples
target_ratio: Desired majority:minority ratio (e.g. 2 for 2:1)

Synthetic samples needed

synthetic_needed = new_minority - current_minority

new_minority: Target minority class size
current_minority: Current minority class size

Frequently Asked Questions

Should I always aim for a 1:1 ratio?

Not necessarily — a moderate target ratio (e.g. 2:1 or 3:1) often generalizes better than forcing perfect balance, since heavy oversampling risks overfitting to synthetic minority patterns.

Does SMOTE just duplicate minority samples?

No — SMOTE generates new synthetic samples by interpolating between existing minority-class neighbors in feature space, rather than simple duplication, which tends to generalize better.

Should oversampling be applied before or after the train/test split?

Always oversample only the training set after splitting — oversampling before splitting leaks synthetic-derived information into the test set and inflates evaluation metrics.

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Undersampling Ratio Calculator Class Imbalance Ratio Calculator Stratified Split Calculator