TryBuildCalcCast-Iron Radiator EDR Sizing Calculator (Equivalent Direct Radiation)
Size your cast-iron radiator instantly.
🕒 Last updated: September 19, 2026
Inputs
ℹ️The AVERAGE of supply and return, not the supply temperature alone — e.g. a 170°F supply with a 150°F return averages to 160°F. Using supply temperature by itself overstates the radiator's real output. Output also rises steeply, not linearly, with this temperature — a modern condensing/modulating boiler run at a low average can cut output well below the classic 170-180°F design rating.
ℹ️Typical planning values at a common ~26 in height — actual EDR varies by manufacturer. Use Custom if you know your radiator's real EDR per section.
ℹ️The overall height, floor to top — including the legs/base, matching how EDR reference charts and manufacturers specify nominal size. EDR per section is scaled from a 26 in reference — an approximation, not a verified per-height table.
ℹ️Count the individual cast-iron slices — each looks like one slice of a loaf of bread. Whole sections only.
ℹ️A room's required heat output, from a load calculation — enter this to check whether the radiator above meets it, and how many sections would.
Heat Output
4,442 BTU/hr
30 sq ft EDR × 148.1 BTU/hr per sq ft (Hot Water at 170°F)
Radiator
Type: 4-Tube, 26 in
Sections: 10
EDR per section: 3 sq ft
Total EDR: 30 sq ft
System
Type: Hot Water
Average water temperature: 170°F
Output rate: 148.1 BTU/hr per sq ft EDR
Assumptions Used
EDR (Equivalent Direct Radiation) is the historic cast-iron radiator sizing standard: 1 sq ft EDR liberates 240 BTU/hr at ~215°F/~1 psi steam and 70°F room air (EDR's own defining condition, confirmed consistently across every source checked). For hot water, output per sq ft EDR follows a power-law curve — BTU/hr = C × (average water temperature − 70°F)^1.3 — using the EN 442 "characteristic curve" exponent industry-standard for panel/cast-iron radiator output (typical range 1.2-1.35), with the constant C calibrated so the curve passes exactly through the universally- agreed 240 BTU/hr steam condition. This average water temperature is the mean of supply and return, not the supply temperature alone. When EDR is determined by tube count and sections, the EDR-per-section figures are typical planning values at a common ~26 in height — real EDR varies meaningfully by manufacturer and casting design (confirmed by professional discussion of this exact issue, since the old IBR independent-testing program was discontinued and manufacturer ratings diverged afterward) — height scaling from the 26 in reference is an approximation, not a verified per-height table. A known actual EDR (nameplate, catalog, or documentation) is always more accurate than these planning defaults. Required EDR and required sections (when a target heat output is entered) are rounded UP, never down, so the displayed figure is never below the true requirement. This is a planning estimate, not a substitute for a licensed heating contractor's design or a specific manufacturer's published rating.
What Is a Cast-Iron Radiator EDR Sizing Calculator?
A cast-iron radiator EDR sizing calculator finds a radiator's heat output using EDR (Equivalent Direct Radiation) — the historic industry standard for rating cast-iron radiators, steam and hot-water boilers, and convectors. It converts between a radiator's physical size (tube count, height, and number of sections) and its BTU/hr output at either a fixed steam temperature or your hot-water system's actual operating temperature, and can also work backward from a required heat output to the number of sections needed.
This calculator is built for homeowners, restorers, and installers working with existing steam or hot-water cast-iron radiator systems — common in older homes across the US. It sizes the radiator itself; it does not size the boiler serving it (a separate calculation with its own piping/pickup allowance — see this site's Boiler / Hydronic Heating Calculator for that), and it does not replace a licensed heating contractor's review for an actual installation decision.
Why this calculator treats the EDR-per-section table as an estimate, not a standard:
- The classic 240 BTU/hr per sq ft EDR steam rate is a single, universally-confirmed figure — no ambiguity there
- Hot-water output varies continuously with average water temperature — this calculator uses a physics-based power-law formula (the EN 442 characteristic-curve exponent) calibrated against the universally-agreed steam condition, rather than a table of hand-picked points
- The EDR-per-section-by-tube-count table is a genuinely non-standardized, manufacturer-varying figure — confirmed by professional discussion of exactly this issue after the old IBR independent-testing program was discontinued — so it's offered as a typical planning default, with a direct "enter your known EDR" option always preferred when available
EDR Formula: How Is It Determined?
Every calculation starts from the radiator's total EDR, then applies a system-specific BTU/hr rate.
Total EDR (By Tube Count & Sections)
EDR per Section (sq ft) = Typical Value at 26 in × (Actual Height ÷ 26)
Total EDR (sq ft) = EDR per Section × Number of Sections
The typical EDR-per-section value depends on tube/column count — a higher tube count packs more radiating surface into the same section. Height scaling from a 26 in reference is an approximation, not a verified per-height table. A known actual EDR (from a nameplate, catalog, or documentation) is always more accurate.
Heat Output — Steam
BTU/hr = Total EDR × 240
Steam radiators run at a fixed, well-defined temperature (~215°F at typical low residential pressure), giving one universal rate.
Heat Output — Hot Water
BTU/hr per Sq Ft EDR = C × (Average Water Temperature − 70°F) ^ 1.3
Output rises with a power (not a straight line) of the temperature difference above room temperature — the 1.3 exponent is the EN 442 "characteristic curve" value used industry-wide for panel/cast-iron radiator output (typical range 1.2-1.35), with the constant C calibrated so this curve passes exactly through the universally-agreed 240 BTU/hr steam condition. Average water temperature is the mean of supply and return — using supply temperature alone overstates the result.
Required Sections for a Target Output (Optional)
Required EDR (sq ft) = Target BTU/hr ÷ BTU/hr per sq ft EDR
Required Sections = ROUND UP(Required EDR ÷ EDR per Section)
Rounds up, never down — a partial section isn't purchasable, and rounding down would undersize the radiator relative to the target.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| System type | Hot Water at 170°F |
| Tube/column type | 4-Tube |
| Height | 26 in |
| Sections | 10 |
Step-by-Step Calculation
| Step | Calculation | Result |
|---|---|---|
| EDR per section | typical × (26 ÷ 26) | 3 sq ft |
| Total EDR | 3 × 10 | 30 sq ft |
| Heat output | 30 × 148.1 | 4,442 BTU/hr |
Therefore, this radiator has 30 sq ft EDR and delivers 4,442 BTU/hr at the specified conditions.
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓EDR Determination+-
- If the radiator's actual EDR is documented anywhere (nameplate, manufacturer catalog, old boiler replacement paperwork), that value was used directly instead of the tube-count table
- Tube or column count correctly identified by looking at the radiator from its narrow end, not guessed from its overall appearance
- Number of sections counted directly on the physical radiator (each cast-iron 'slice'), not estimated from its overall width
- Confirmed the radiator being sized is a floor-standing column or tube type, not a flat wall-hung (wall type) radiator
✓System Type & Temperature+-
- System type (steam vs. hot water) matches the actual heating system, not assumed from the radiator's appearance alone
- For a hot-water system, the entered average water temperature reflects the system's actual current operating temperature (boiler aquastat/reset curve setting), not an assumed historical default
- Confirmed the entered water temperature is the AVERAGE of supply and return, not the boiler's supply temperature alone
✓Target & Sizing Decisions+-
- If a target heat output was entered, it came from an actual room heat-loss/load calculation, not a guess
- Understood that the required-sections figure is rounded UP to the next whole section, and that ordering exactly that many (not fewer) is what actually meets the target
- Confirmed this calculator's output sizes the RADIATOR, not the boiler serving it — a separate boiler-sizing calculation (with its own piping/pickup allowance) still applies
✓Physical Verification+-
- For any radiator replacement, relocation, or new boiler sizing decision, a licensed heating contractor was consulted rather than relying on this calculator alone
Full QC Checklist+−
Verification checklist for a cast-iron radiator EDR (Equivalent Direct Radiation) sizing estimate — covering EDR determination, system type and temperature, target and sizing decisions, and physical verification. Use the Essential Checklist for critical checks before finalizing, expand to Full QC Checklist for complete verification.
✓EDR Determination+-
- If the radiator's actual EDR is documented anywhere (nameplate, manufacturer catalog, old boiler replacement paperwork), that value was used directly instead of the tube-count table
- Tube or column count correctly identified by looking at the radiator from its narrow end, not guessed from its overall appearance
- Number of sections counted directly on the physical radiator (each cast-iron 'slice'), not estimated from its overall width
- Understood that height scaling from the 26 in reference is an approximation, not a verified per-height table, for a radiator meaningfully taller or shorter than ~26 in
- Confirmed the radiator being sized is a floor-standing column or tube type, not a flat wall-hung (wall type) radiator
- Radiator height measured as the overall height, floor to top — including the legs/base, not just the cast-iron body above them
✓System Type & Temperature+-
- System type (steam vs. hot water) matches the actual heating system, not assumed from the radiator's appearance alone
- For a hot-water system, the entered average water temperature reflects the system's actual current operating temperature (boiler aquastat/reset curve setting), not an assumed historical default
- Confirmed the entered water temperature is the AVERAGE of supply and return, not the boiler's supply temperature alone
- If retrofitting a condensing or modulating boiler onto existing cast-iron radiators, accounted for the real possibility of a large (up to ~two-thirds) output reduction at a low average water temperature
- Confirmed this radiator's actual piping doesn't mix steam and hot-water distribution — the two systems use different pipe sizing, venting, and pitch requirements entirely
✓Target & Sizing Decisions+-
- If a target heat output was entered, it came from an actual room heat-loss/load calculation, not a guess
- Understood that the required-sections figure is rounded UP to the next whole section, and that ordering exactly that many (not fewer) is what actually meets the target
- Confirmed this calculator's output sizes the RADIATOR, not the boiler serving it — a separate boiler-sizing calculation (with its own piping/pickup allowance) still applies
- If a room has more than one radiator, each was sized separately and their outputs summed, rather than treating the room as having a single combined radiator
- If comparing this calculator's output against an old boiler's nameplate rating, confirmed which basis that nameplate figure actually uses (NET vs. GROSS I=B=R), not assumed
✓Physical Verification+-
- Radiator confirmed unobstructed by furniture, drapes, or covers that would reduce its real delivered output below this calculation
- For a hot-water radiator, air bleed valve checked and system properly purged of air, and for a steam radiator, the air vent checked for proper operation
- For any radiator replacement, relocation, or new boiler sizing decision, a licensed heating contractor was consulted rather than relying on this calculator alone
- For a steam system, each radiator's own air vent checked for a venting rate appropriate to that radiator's actual EDR, not left as a generic one-size-fits-all vent
Radiator EDR Reference Tables
BTU/hr per sq ft EDR by system type/average water temperature (computed from this calculator's own formula, not a separate table), and typical EDR per section at 26 in by tube count.
| System | BTU/hr per Sq Ft EDR |
|---|---|
| Steam (~215°F) | 240 |
| Hot water at 120°F average | 60.1 |
| Hot water at 140°F average | 93.1 |
| Hot water at 150°F average | 110.8 |
| Hot water at 160°F average | 129.1 |
| Hot water at 170°F average | 148.1 |
| Hot water at 180°F average | 167.6 |
| Hot water at 200°F average | 208.2 |
Typical EDR per Section at 26 in (Planning Estimate)
| Tube/Column Count | Typical Sq Ft EDR per Section |
|---|---|
| 2-Tube | 2 |
| 3-Tube | 2.25 |
| 4-Tube | 3 |
| 5-Tube | 3.5 |
| 6-Tube | 4 |
When should you use this radiator EDR calculator?
- Finding an existing cast-iron radiator's real heat output, to check it against a room's actual heat loss.
- Sizing a replacement or additional radiator for a room, given a target heat output from a load calculation.
- Checking whether existing radiators still deliver enough heat at a condensing/modulating boiler's lower average water temperature before a boiler replacement.
- Converting between a documented EDR figure and BTU/hr output for boiler-sizing input (feeding into this site's Boiler / Hydronic Heating Calculator).
- Comparing two scenarios side by side (via Compare mode) — e.g. a radiator's output at its classic design temperature vs. a planned lower condensing-boiler temperature.
Quick Sizing Tips
- Always prefer a documented actual EDR (nameplate, catalog, old paperwork) over the tube-count table's typical planning defaults — real EDR varies meaningfully by manufacturer.
- Use your system's actual current operating water temperature, not an assumed 170-180°F historical default — check the boiler's aquastat or reset curve.
- Before a condensing/modulating boiler retrofit, re-run this calculator at the planned low average water temperature — don't assume the radiators' classic rating still applies.
- Round up to the next whole section when sizing for a target — a partial section isn't purchasable, and rounding down undersizes the radiator.
- For a room with multiple radiators, run this calculator once per radiator and sum the outputs — it models one radiator at a time.
- Rule out trapped air, blocked airflow, and a lower-than-assumed operating temperature before concluding a radiator is genuinely undersized.
Common Mistakes
- Treating the tube-count table's EDR-per-section value as a fixed universal standard instead of a manufacturer-varying planning estimate.
- Using the classic 170-180°F hot-water design rating for a system actually running at a lower, modern condensing/modulating temperature.
- Confusing radiator sizing (this calculator) with boiler sizing (a separate calculation with its own piping/pickup allowance).
- Miscounting tube or column count by guessing from the radiator's overall look instead of checking from its narrow end.
- Rounding a fractional required-section count down instead of up, undersizing the radiator relative to its target.
- Sizing a room with multiple radiators using only one radiator's output instead of summing all of them.
- Overlooking trapped air, blocked airflow, or an actual operating temperature lower than assumed when a radiator seems undersized.
Limitations
- This is a planning estimate based on cross-verified EDR conventions, not a substitute for a documented actual EDR rating or a licensed heating contractor's design and review.
- The EDR-per-section-by-tube-count table is a typical planning default, not a universal standard — real EDR varies by manufacturer and casting design; a documented actual value is always preferred.
- Height scaling from the 26 in reference is an approximation checked against one manufacturer's multi-height data (within ~10-15%), not a verified per-height table for every manufacturer.
- Hot-water output uses a 1.3 characteristic-curve exponent, the EN 442 industry-typical value (real radiators can vary roughly 1.2-1.35) — not a value independently verified for every specific cast-iron radiator model.
- Requires the system's AVERAGE water temperature (the mean of supply and return) — entering a supply temperature alone overstates the calculated output.
- Does not model wall-type (flat, wall-hung) cast-iron radiators, only floor-standing column/tube types — wall-type radiators use a different height/width/thickness-based EDR table.
- Does not size the boiler serving the radiator — boiler sizing needs a separate piping/pickup allowance this calculator doesn't include.
- Does not model more than one radiator per calculation — for a room with multiple radiators, run this calculator once per radiator and sum the results.
- Does not account for trapped air, obstructed airflow, or other real-world factors that reduce a radiator's actual delivered output below its calculated EDR-based figure.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.