Half-life describes how quickly a compound's activity declines after an entry. It is the core variable behind every accumulation estimate. Understanding it helps you interpret what the accumulation calculator is showing — and why the same entry amount can produce very different buildup patterns depending on the compound.
What Half-Life Means
Half-life is the time it takes for an estimated 50% of a compound's activity to decline. After one half-life, roughly half remains active. After two half-lives, about a quarter remains. This decline continues with each passing half-life until activity becomes negligible.
Starting activity: 100 units
After 1 half-life: ~50 units remaining
After 2 half-lives: ~25 units remaining
After 3 half-lives: ~12.5 units remaining
After 4 half-lives: ~6 units remaining
After 5 half-lives: ~3 units remaining
Shorter vs. Longer Half-Lives
A compound with a short half-life declines quickly. Less activity carries into the next interval, which means less overlap and a lower steady-state level. A compound with a long half-life declines slowly. More activity carries over, creating more overlap and higher accumulation over time.
This is why two compounds at the same entry amount can produce completely different accumulation curves — half-life drives the difference.
How Repeated Intervals Create Buildup
When a new entry is introduced before the prior one has fully declined, the remaining activity stacks with the incoming amount. Over several intervals, this carryover compounds — levels climb higher until the amount declining per interval equals the amount being added. That balance point is steady state.
Interval 1: 100 units introduced → 100 active at peak
Interval 2: 50 carry over + 100 new → 150 active at peak
Interval 3: 75 carry over + 100 new → 175 active at peak
Interval 6+: carryover stabilizes → steady state reached
Built-In Estimated Half-Life Values
The accumulation calculator includes built-in estimated half-life values for common research compounds. When you select a compound, the half-life field auto-fills with a research-based estimate — no manual lookup needed.
These estimates are designed to give you a practical, accurate starting point for modeling. They reflect typical activity patterns and are regularly maintained to stay current.
Custom Override
If you want to model a specific half-life value — or work with a compound not included in the built-in library — you can manually enter a custom value. The custom entry replaces the auto-filled estimate for that calculation.
Custom overrides are useful for comparative modeling, exploring how different half-life assumptions affect accumulation curves, or working with newer compounds not yet in the library.
Why the Output Is an Estimate
Half-life values — whether built-in or custom — are averages derived from research data. Actual activity in any individual system is affected by factors the calculator cannot model. All accumulation outputs should be read as informed estimates, not precise measurements.
Quick Reference
Half-life = time for ~50% of activity to decline
Shorter half-life = faster decline, less carryover, lower accumulation
Longer half-life = slower decline, more carryover, higher accumulation
Steady state = when carryover per interval equals amount introduced
Built-in estimates auto-fill — custom override available for any compound
Common Mistakes
Half-life drives accumulation behavior. Two compounds at the same entry amount will accumulate very differently if their half-lives differ.
Built-in values are research-based and accurate for most modeling purposes. Only override if you have a specific value to test. An inaccurate custom entry distorts the entire output.
Steady state typically takes multiple half-life cycles to approach. Review the full accumulation curve in the output rather than looking only at early intervals.
This guide is for research-use calculator education only. It does not provide medical advice, treatment recommendations, or personalized dosing instructions.