Enter immobility, swimming, and climbing times per animal. Get % immobility, time-bin breakdown, behavior proportions, and group comparisons.
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The first 1-2 minutes of the FST session are dominated by vigorous escape behavior in all animals, regardless of treatment. Including this period dilutes between-group differences. Score only the designated test interval (e.g., minutes 2-6 of a 6-minute session, or the full 5 minutes of a 5-minute session).
Water temperature must be tightly controlled at 25 +/- 1 degrees Celsius. Warmer water (>28 degrees) increases immobility; cooler water (<23 degrees) decreases it and increases active behaviors. A 2-3 degree temperature shift can produce effects comparable to antidepressant treatment. Use a thermometer and adjust before each animal.
Immobility + swimming + climbing should equal total scored time. A discrepancy greater than 5-10% of session duration suggests scoring errors, gaps, or overlapping behavior codes. This calculator flags discrepancies exceeding 10%. Review flagged animals before drawing conclusions.
Observer bias can substantially influence manual FST scoring — knowledge of treatment group shifts immobility estimates by 10-20%. All scoring should be performed blind to treatment condition. If using automated scoring, validate against blinded manual scoring in a subset of animals.
Female rodents often show lower baseline immobility and different antidepressant response profiles than males. Analyze sexes separately or include sex as a factor. The NIH mandate to consider sex as a biological variable applies directly to FST studies.
Percent immobility equals immobility time divided by total scored session duration, multiplied by 100. Behavior proportions are computed as each behavior time divided by total scored time. For time-bin mode, each 5-second bin is classified by predominant behavior, and immobility counts are converted to cumulative immobility curves. Latency to first immobility is the start time of the first bin scored as immobility. Group statistics use sample standard deviation (n-1 denominator) for SEM computation. All computation is client-side — no data leaves your browser.
Last validated 2026-04-05. Calculations are designed for planning and documentation support; verify procurement decisions against manufacturer specifications or institutional SOPs.
ConductScience Forced Swim Test Immobility Calculator (v1.0). ConductScience, Inc. 2026. Available at: https://conductscience.com/tools/forced-swim-test-immobility-calculator
This tool performs mathematical calculations on user-provided data. It does not replace scientific judgment regarding experimental design, exclusion criteria, or statistical analysis.
The forced swim test (FST), introduced by Porsolt, Le Pichon, and Jalfre (1977), is one of the most widely used preclinical assays for antidepressant-like activity. A mouse or rat is placed in an inescapable cylinder of water (typically 25 +/- 1 degrees Celsius) for a fixed duration. After initial vigorous escape attempts (swimming and climbing), the animal progressively adopts an immobile posture — floating passively with only minor movements to maintain its head above water. This transition to immobility is interpreted as behavioral despair or passive coping, and it is sensitive to antidepressant drugs: acute or subchronic administration of tricyclics, SSRIs, SNRIs, and other antidepressants reliably reduces immobility. The test is valued for its simplicity, high throughput, and pharmacological predictive validity. However, interpretation remains debated — some researchers argue immobility reflects an adaptive energy-conservation strategy rather than despair. Regardless of interpretive framework, the FST remains a standard screening tool in antidepressant drug discovery and a common phenotyping assay for transgenic models of depression.
Immobility in the FST is defined as the absence of active escape-oriented movements. The animal floats in the water, making only small adjustments necessary to keep its nose above the surface. This is distinct from swimming (active horizontal movement through the water) and climbing (vigorous upward-directed movements with forepaws breaking the water surface against the cylinder wall). Scoring can be performed manually by a trained observer watching recorded video, using either continuous timing (stopwatch for each behavior) or time-sampling (predominant behavior scored in consecutive 5-second bins). Automated scoring systems detect immobility from video using pixel-change, centroid velocity, or pose-estimation algorithms. Manual scoring requires blinding and inter-rater reliability assessment (Cohen kappa > 0.85 is considered acceptable). Whether manual or automated, the same behavioral definitions must be applied consistently. The scored interval is typically the last 4 minutes of a 6-minute session (rats) or the full 6 minutes (mice), though protocols vary by laboratory.
The modified forced swim test, developed by Detke, Rickels, and Lucki (1995), introduced the three-behavior scoring system that distinguishes immobility, swimming, and climbing. This modification was motivated by the observation that different classes of antidepressants produce distinct behavioral profiles: serotonergic drugs (fluoxetine, sertraline) selectively increase swimming behavior, while noradrenergic drugs (desipramine, reboxetine) selectively increase climbing behavior. Both reduce immobility, but the active behavior that replaces immobility differs. This pharmacological dissociation provides additional information beyond total immobility: if a novel compound reduces immobility by increasing swimming, it likely has serotonergic activity; if by increasing climbing, noradrenergic activity. The three behaviors are mutually exclusive and should sum to the total scored duration. Scoring is typically done in 5-second bins with the predominant behavior recorded for each bin, yielding counts that are converted to time (count times 5 seconds) or percentages. This approach has been validated across multiple laboratories and is now standard in most FST protocols.
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