Compare peak inflow, zone shortfalls and drainage-system counts using verified rainfall, effective catchments and manufacturer design capacities.
Compare roof inflow with each drainage zone in L/s.
Use non-overlapping catchments and design capacities matching the outlet, run length and gradient. Counts assume equal catchment allocation to independent systems.
Source check: 2026-09-29 · SI units; verify site rainfall and product design tables
Do not enter monthly rainfall depth (mm). Blank means unknown; zero means no rainfall.
Total assumed peak inflow
N/A L/s
Sum of zone shortfalls
N/A L/s
Zones awaiting evidence
1 / 1
Inflow conversion: N/A L/min · N/A m³/h
Displays are rounded; comparisons use unrounded values. Zero inflow is not a drainage approval. Threshold intensity is an arithmetic equality, not a local design rainfall recommendation.
| Scenario | mm/h | Inflow L/s | Shortfall L/s |
|---|---|---|---|
| 80% | N/A | N/A | N/A |
| ★ 100% | N/A | N/A | N/A |
| 120% | N/A | N/A | N/A |
80/100/120% are neither probabilities nor recommended design values. Spare capacity in another zone does not reduce a shortfall.
Next step: share roof plans, rainfall evidence and current product tables.
A qualified designer should check catchment splits, outlet layout, shared pipes, backwater and emergency overflow. This comparison does not certify hydraulic design, structure, waterproofing, code compliance or flood protection.
Roof runoff and gutter capacity comparison Source check: 2026-09-29 User input scenario Q = C × I × A / 3600; q_use = q × (1 − b/100); K = n × q_use; ceil(Q/q_use) Rainfall: N/A mm/h | | | Unconfirmed Zone 1: A=N/A m²; effective; C=1; Unconfirmed q=N/A L/s; n=1; b=0% Product: | | Unconfirmed Layout: | Independent equal allocation: Unconfirmed Comparison pending: Confirm rainfall, source and document date; Confirm effective area, slope, wind and runoff coefficient; Confirm design capacity, product source, date and layout; Confirm identical independent systems and equal catchment allocation Q=N/A L/s; N/A L/min; N/A m³/h q_use=N/A L/s; K=N/A L/s; Margin=N/A L/s; Shortfall=N/A L/s Candidate systems / additional: N/A / N/A; Utilization=N/A%; Threshold intensity=N/A mm/h Total assumed inflow / sum of zone shortfalls: N/A / N/A L/s 80% (N/A mm/h): Inflow / shortfall N/A / N/A L/s Zone 1: Q=N/A L/s; Comparison pending; Candidates=N/A 100% (N/A mm/h): Inflow / shortfall N/A / N/A L/s Zone 1: Q=N/A L/s; Comparison pending; Candidates=N/A 120% (N/A mm/h): Inflow / shortfall N/A / N/A L/s Zone 1: Q=N/A L/s; Comparison pending; Candidates=N/A Counts assume equal catchment allocation to identical independent systems. Shared downstream pipes, backwater, siphonic operation, emergency overflow, structure, waterproofing and code compliance are outside this comparison. Spare capacity never offsets another zone’s shortfall. 80/100/120% are sensitivity scenarios, not probabilities or design recommendations. Ask a qualified designer to review roof plans, rainfall evidence and current product tables.
Overflowing eaves often prompt a quote for larger gutters or downpipes.
The relevant comparison also depends on the catchment, rainfall intensity, outlet location, gutter length and gradient.
This calculator helps homeowners, families and small-building managers assemble a consistent comparison before accepting a replacement quote.
It identifies the zone and numerical shortfall to discuss with a drainage designer.
A tank’s storage volume and a drainage system’s flow capacity answer different questions.
A large tank cannot capture water that spills before reaching its inlet.
This page compares peak inflow with drainage capacity in SI units; use the rainwater storage calculator for monthly water balance and water-bill savings.
Enter intensity in mm/h, with the source, location, return period, duration and document date.
Monthly or daily rainfall depth cannot be used directly as peak intensity.
Confirm that the supplied design intensity applies to the site.
Use the effective area draining to this zone.
Check how slope, wind-driven rain and wall runoff have been treated.
Select horizontal plan area only if that is all you know.
The starting coefficient is not a recommendation for every roof material or green roof.
Record the manufacturer, model, source and revision date, together with outlet position, run length and gradient.
Enter usable design capacity for those conditions, not an unadjusted brim-full maximum.
Capacities of components connected in series must not be added.
Count identical independent systems and confirm equal catchment allocation.
Any additional capacity reduction is a user scenario for uncertainty.
It is neither a statutory safety factor nor a hydraulic conversion from the percentage of physically blocked area.
Area starts at 0.01 m², and each roof portion should appear in only one catchment.
You can enter up to six zones; different products or outlet conditions belong in separate zones.
A blank value means unknown and is distinct from an explicitly entered zero.
Q = C × I × A / 3600
Q is in L/s, C is the runoff coefficient, I is in mm/h and A is effective area in m².
One millimetre over one square metre is one litre, so dividing the hourly volume by 3600 converts it to seconds.
Multiply Q by 60 for L/min or by 3.6 for m³/h.
q_use = q × (1 − b/100), K = n × q_use.
q is design capacity per system, b is the additional reduction percentage and n is the existing system count.
Margin is K−Q and shortfall is max(Q−K, 0).
A positive margin is only a numerical difference under the entered assumptions, not installation approval or flood protection.
Candidate systems = ceil(Q/q_use); additional candidates = max(candidates−n, 0).
Rounding upward assumes identical independent systems can receive equal catchment areas.
Intensity at capacity is 3600K/(C×A).
This arithmetic equality must not be used to select a lower local design rainfall intensity.
For 100 m² effective area, 100 mm/h rainfall and C=1, peak inflow is approximately 2.778 L/s.
One system rated at 2 L/s with no additional reduction has 2 L/s capacity.
The shortfall is approximately 0.778 L/s, giving two candidate systems and one additional candidate.
The intensity at capacity is 72 mm/h.
The feasibility of equal catchment division and the new layout’s product rating still require review.
With two existing systems and a 25% additional reduction, each provides 1.5 L/s and the combined zone capacity is 3 L/s.
The numerical margin is approximately 0.222 L/s.
That does not establish that a shared downstream pipe or outlet has adequate capacity.
Returning to one system, 80% rainfall means 80 mm/h and 2.222 L/s inflow.
At 120%, 120 mm/h produces 3.333 L/s.
These are fictional values for checking units and formulas, not local rainfall recommendations or quoted product performance.
Displays are rounded while comparisons use unrounded values.
Very small nonzero values may use scientific notation, so similar-looking rounded numbers can still have a real shortfall.
Zero existing systems can still produce a candidate count when the verified per-system capacity is positive.
The table compares 80/100/120% of entered rainfall and shows total inflow and summed zone shortfalls.
These multipliers are sensitivity assumptions, not return periods, climate allowances or probabilities.
A designer must select the applicable site intensity from appropriate evidence and criteria.
If zone A has a shortfall and zone B has ample spare capacity, the total still includes A’s shortfall.
Water does not automatically move between catchments without changing the physical drainage arrangement.
If any zone lacks comparison evidence, the total shortfall stays N/A to prevent a whole-building conclusion from incomplete data.
Before dividing total roof area by downpipe count, identify the actual area feeding the overflowing section.
Separate zones expose local shortfalls hidden by a building-wide total.
Review debris, sagging and connection defects alongside the calculation.
Request each quote’s outlet layout, gradient and capacity source as well as the proposed quantity and size.
Compare them using the same rainfall and catchment assumptions, saving a report for each arrangement.
If a cheaper quote has a greater shortfall, discuss whether the quantity or layout needs to change.
This tool does not derive pipe capacity from diameter, length, roughness or hydraulic head.
Siphonic systems, pressurized flow, shared downstream pipes, backwater and emergency overflow need separate design work.
A numerical margin also says nothing definitive about roof loads, waterproofing, brackets, corrosion or the actual effect of cleaning.
Do not invent a roof-pitch correction or assume a larger pipe diameter establishes capacity.
Do not replace a reduced design rating with the manufacturer’s brim-full maximum.
Check where overflowing water would travel and whether inspection and cleaning access are practical before agreeing the construction scope.
Monthly rainfall is an accumulated depth.
The calculation requires intensity in mm/h for the applicable design duration and return period, so monthly depth cannot be entered directly.
Select horizontal plan area only.
Provisional inflow remains visible, but capacity comparison and counts stay pending until effective area and treatment of slope, wind and wall runoff are confirmed.
This calculator does not convert diameter to flow.
Enter a manufacturer design rating matching the gutter shape, run length, outlet position and connections.
The count assumes equal catchment allocation to identical independent systems.
Two outlets feeding one undersized downstream pipe can still have a bottleneck, so the revised layout requires review.
There is no universal recommended percentage here.
Check allowances already included in the rating and document your maintenance scenario.
Blocked surface area does not necessarily equal flow loss.
It only indicates spare numerical capacity for the entered scenario.
More intense rain, debris, sagging, backwater and downstream restrictions remain relevant, along with the emergency overflow route.
They make the assumptions traceable when layouts or product documents change.
Missing sources or dates keep the comparison pending even with confirmation boxes selected.
A date alone does not certify a document’s applicability.
Both interfaces use the same inputs and pure calculation functions in SI units.
Saved reports retain zone values, sources, confirmation states and whether the inputs started from the fictional example.
Source review: 2026-09-29.
The physical comparison is general, while the cited legal context is Korean and is not a compliance determination for any jurisdiction.
No national rainfall default or statutory downpipe-size table is supplied.
Recheck applicable local design criteria, rainfall duration and frequency, and current product revisions at the time of use.
Enter actual evidence and resolve the listed pending items first.
Mark the deficient zones and proposed system changes on your plan, then share the saved report so the contractor can compare the same assumptions.
Saving or printing does not submit an enquiry or approve a design; agree the final arrangement and construction scope with the reviewer.