diff --git a/packages/domain/src/domain/sap/calculator.py b/packages/domain/src/domain/sap/calculator.py
new file mode 100644
index 00000000..881c8931
--- /dev/null
+++ b/packages/domain/src/domain/sap/calculator.py
@@ -0,0 +1,274 @@
+"""SAP 10.3 synthetic-input calculator orchestrator.
+
+Drives the 12-month heat-balance loop from a typed `CalculatorInputs`
+aggregate and emits a typed `SapResult`. This module is the physics
+assembly only — the RdSAP cert→inputs mapping lives in
+`domain.sap.rdsap.cert_to_inputs` (Session A slice 7b). Splitting the two
+keeps orchestration testable against synthetic inputs without dragging in
+cert-shape assumptions.
+
+Each month:
+
+  1. External temperature, wind speed, horizontal solar irradiance from
+     Appendix U Tables U1-U3 by region + month.
+  2. Internal gains (§5 + Appendix L) given TFA and month.
+  3. Solar gains (§6 + Appendix U §U3.2) summed over the window list.
+  4. HLC = HLC_T (already supplied) + HLC_V = ach × volume × 0.33.
+  5. Thermal time constant τ = TMP × TFA / (3.6 × HLC) for utilisation η.
+  6. Mean internal temperature (§7 + Table 9b/9c) and utilisation factor
+     (Table 9a) — iterated twice because each depends on the other; SAP
+     10.3 §7.3 says two passes are sufficient.
+  7. Useful space-heating requirement (Table 9c step 10).
+  8. Delivered fuel kWh = Q_heat / main-heating efficiency.
+
+Annual totals = month sums; ECF = §13 Table 12 deflator × total cost /
+(TFA + 45); SAP rating from §13 piecewise log/linear; CO2 from CO2
+emission factor × delivered fuel (single-fuel approximation in this
+slice — slice S-A8 splits hot-water/lighting onto per-fuel factors).
+
+Reference: SAP 10.3 specification (13-01-2026) §§5-13 (pages 23-43), Table
+9a/9b/9c (pages 184-186), Table 12 (page 191), Appendix L + U.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Final
+
+from domain.sap.climate.appendix_u import external_temperature_c
+from domain.sap.worksheet.dimensions import Dimensions
+from domain.sap.worksheet.heat_transmission import HeatTransmission
+from domain.sap.worksheet.internal_gains import internal_gains_w
+from domain.sap.worksheet.mean_internal_temperature import mean_internal_temperature_c
+from domain.sap.worksheet.rating import energy_cost_factor, sap_rating, sap_rating_integer
+from domain.sap.worksheet.solar_gains import (
+    Orientation,
+    surface_solar_flux_w_per_m2,
+    window_solar_gain_w,
+)
+from domain.sap.worksheet.space_heating import monthly_heat_requirement_kwh
+from domain.sap.worksheet.utilisation_factor import utilisation_factor
+
+
+_DAYS_IN_MONTH: Final[tuple[int, ...]] = (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
+_AIR_HEAT_CAPACITY_WH_PER_M3_K: Final[float] = 0.33
+_TIME_CONSTANT_DIVISOR_KJ_TO_WH: Final[float] = 3.6
+_ETA_ITERATIONS: Final[int] = 2
+
+
+@dataclass(frozen=True)
+class WindowInput:
+    """One glazed opening contributing solar gain. Orientation maps to a
+    Table U5 column and to Table U4 latitude via `surface_solar_flux_w_per_m2`.
+    `g_perpendicular`, `frame_factor`, `overshading_factor` come from
+    Tables 6b/6c/6d — supplied by the caller so this module remains
+    physics-only."""
+
+    area_m2: float
+    orientation: Orientation
+    pitch_deg: float
+    g_perpendicular: float
+    frame_factor: float
+    overshading_factor: float
+
+
+@dataclass(frozen=True)
+class CalculatorInputs:
+    """Synthetic SAP 10.3 calculator inputs. The cert→inputs mapper
+    (S-A7b) produces one of these from an `EpcPropertyData`."""
+
+    dimensions: Dimensions
+    heat_transmission: HeatTransmission
+    infiltration_ach: float
+    region: int
+    windows: tuple[WindowInput, ...]
+    control_type: int
+    responsiveness: float
+    living_area_fraction: float
+    control_temperature_adjustment_c: float
+    thermal_mass_parameter_kj_per_m2_k: float
+    main_heating_efficiency: float
+    hot_water_kwh_per_yr: float
+    pumps_fans_kwh_per_yr: float
+    lighting_kwh_per_yr: float
+    fuel_unit_cost_gbp_per_kwh: float
+    co2_factor_kg_per_kwh: float
+
+
+@dataclass(frozen=True)
+class MonthlyEntry:
+    """Per-month worksheet outputs for downstream audit. SAP 10.3 §§5-9."""
+
+    month: int
+    external_temp_c: float
+    internal_temp_c: float
+    internal_gains_w: float
+    solar_gains_w: float
+    heat_loss_rate_w: float
+    utilisation_factor: float
+    space_heat_requirement_kwh: float
+    main_heating_fuel_kwh: float
+
+
+@dataclass(frozen=True)
+class SapResult:
+    """Calculator output. `sap_score` is the rounded RdSAP-style integer
+    (1-100+); `sap_score_continuous` keeps the un-rounded value for
+    sensitivity analysis."""
+
+    sap_score: int
+    sap_score_continuous: float
+    ecf: float
+    total_fuel_cost_gbp: float
+    co2_kg_per_yr: float
+    space_heating_kwh_per_yr: float
+    main_heating_fuel_kwh_per_yr: float
+    hot_water_kwh_per_yr: float
+    pumps_fans_kwh_per_yr: float
+    lighting_kwh_per_yr: float
+    monthly: tuple[MonthlyEntry, ...]
+
+
+def _solar_gains_w(
+    *, windows: tuple[WindowInput, ...], region: int, month: int
+) -> float:
+    total = 0.0
+    for w in windows:
+        s = surface_solar_flux_w_per_m2(
+            orientation=w.orientation,
+            pitch_deg=w.pitch_deg,
+            region=region,
+            month=month,
+        )
+        total += window_solar_gain_w(
+            area_m2=w.area_m2,
+            surface_flux_w_per_m2=s,
+            g_perpendicular=w.g_perpendicular,
+            frame_factor=w.frame_factor,
+            overshading_factor=w.overshading_factor,
+        )
+    return total
+
+
+def _time_constant_h(*, tmp_kj_per_m2_k: float, tfa_m2: float, hlc_w_per_k: float) -> float:
+    if hlc_w_per_k <= 0:
+        return float("inf")
+    return tmp_kj_per_m2_k * tfa_m2 / (_TIME_CONSTANT_DIVISOR_KJ_TO_WH * hlc_w_per_k)
+
+
+def _solve_month(
+    *,
+    inputs: CalculatorInputs,
+    month: int,
+    hlc_w_per_k: float,
+    time_constant_h: float,
+    heat_loss_parameter: float,
+) -> MonthlyEntry:
+    t_ext = external_temperature_c(inputs.region, month)
+    g_int = internal_gains_w(
+        total_floor_area_m2=inputs.dimensions.total_floor_area_m2,
+        month=month,
+    ).total_w
+    g_sol = _solar_gains_w(windows=inputs.windows, region=inputs.region, month=month)
+    g_total = g_int + g_sol
+
+    # SAP 10.3 §7.3: two-pass iteration. Seed η = 1, compute T_internal,
+    # recompute η from the resulting loss rate, then once more.
+    eta = 1.0
+    t_int = 0.0
+    loss_rate_w = 0.0
+    for _ in range(_ETA_ITERATIONS):
+        t_int = mean_internal_temperature_c(
+            external_temp_c=t_ext,
+            heat_transfer_coefficient_w_per_k=hlc_w_per_k,
+            total_gains_w=g_total,
+            utilisation_factor=eta,
+            time_constant_h=time_constant_h,
+            heat_loss_parameter=heat_loss_parameter,
+            living_area_fraction=inputs.living_area_fraction,
+            control_type=inputs.control_type,
+            responsiveness=inputs.responsiveness,
+            control_temperature_adjustment_c=inputs.control_temperature_adjustment_c,
+        )
+        loss_rate_w = max(0.0, hlc_w_per_k * (t_int - t_ext))
+        eta = utilisation_factor(
+            total_gains_w=g_total,
+            heat_loss_rate_w=loss_rate_w,
+            time_constant_h=time_constant_h,
+        )
+
+    q_heat = monthly_heat_requirement_kwh(
+        heat_transfer_coefficient_w_per_k=hlc_w_per_k,
+        internal_temperature_c=t_int,
+        external_temperature_c=t_ext,
+        utilisation_factor=eta,
+        total_gains_w=g_total,
+        days_in_month=_DAYS_IN_MONTH[month - 1],
+    )
+    fuel = q_heat / inputs.main_heating_efficiency if inputs.main_heating_efficiency > 0 else 0.0
+
+    return MonthlyEntry(
+        month=month,
+        external_temp_c=t_ext,
+        internal_temp_c=t_int,
+        internal_gains_w=g_int,
+        solar_gains_w=g_sol,
+        heat_loss_rate_w=loss_rate_w,
+        utilisation_factor=eta,
+        space_heat_requirement_kwh=q_heat,
+        main_heating_fuel_kwh=fuel,
+    )
+
+
+def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
+    """Run SAP 10.3 §§5-13 monthly loop on synthetic inputs; return a
+    typed `SapResult`. Cert-shape mapping is the job of `cert_to_inputs`
+    (S-A7b); this entry point is pure physics."""
+    tfa = inputs.dimensions.total_floor_area_m2
+    hlc_v = inputs.infiltration_ach * inputs.dimensions.volume_m3 * _AIR_HEAT_CAPACITY_WH_PER_M3_K
+    hlc = inputs.heat_transmission.total_w_per_k + hlc_v
+    hlp = hlc / tfa if tfa > 0 else 0.0
+    tau_h = _time_constant_h(
+        tmp_kj_per_m2_k=inputs.thermal_mass_parameter_kj_per_m2_k,
+        tfa_m2=tfa,
+        hlc_w_per_k=hlc,
+    )
+
+    monthly = tuple(
+        _solve_month(
+            inputs=inputs,
+            month=m,
+            hlc_w_per_k=hlc,
+            time_constant_h=tau_h,
+            heat_loss_parameter=hlp,
+        )
+        for m in range(1, 13)
+    )
+
+    space_heating_kwh = sum(e.space_heat_requirement_kwh for e in monthly)
+    main_fuel_kwh = sum(e.main_heating_fuel_kwh for e in monthly)
+    delivered_fuel_kwh = (
+        main_fuel_kwh
+        + inputs.hot_water_kwh_per_yr
+        + inputs.pumps_fans_kwh_per_yr
+        + inputs.lighting_kwh_per_yr
+    )
+    total_cost = delivered_fuel_kwh * inputs.fuel_unit_cost_gbp_per_kwh
+    ecf = energy_cost_factor(total_cost_gbp=total_cost, total_floor_area_m2=tfa)
+    sap_int = sap_rating_integer(ecf=ecf)
+    sap_cont = sap_rating(ecf=ecf)
+    co2 = delivered_fuel_kwh * inputs.co2_factor_kg_per_kwh
+
+    return SapResult(
+        sap_score=sap_int,
+        sap_score_continuous=sap_cont,
+        ecf=ecf,
+        total_fuel_cost_gbp=total_cost,
+        co2_kg_per_yr=co2,
+        space_heating_kwh_per_yr=space_heating_kwh,
+        main_heating_fuel_kwh_per_yr=main_fuel_kwh,
+        hot_water_kwh_per_yr=inputs.hot_water_kwh_per_yr,
+        pumps_fans_kwh_per_yr=inputs.pumps_fans_kwh_per_yr,
+        lighting_kwh_per_yr=inputs.lighting_kwh_per_yr,
+        monthly=monthly,
+    )
diff --git a/packages/domain/src/domain/sap/tests/__init__.py b/packages/domain/src/domain/sap/tests/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/packages/domain/src/domain/sap/tests/test_calculator.py b/packages/domain/src/domain/sap/tests/test_calculator.py
new file mode 100644
index 00000000..b53b573b
--- /dev/null
+++ b/packages/domain/src/domain/sap/tests/test_calculator.py
@@ -0,0 +1,196 @@
+"""Tests for the synthetic-input Sap10 calculator orchestrator.
+
+The orchestrator drives SAP 10.3's 12-month heat-balance loop from a
+`CalculatorInputs` aggregate (geometry, envelope, ventilation, climate,
+heating + the running-cost lines hot-water/pumps-fans/lighting). It
+returns a typed `SapResult` carrying the SAP score, the cost/CO2 totals,
+and a 12-entry `monthly` breakdown so downstream consumers can audit
+month-by-month physics.
+
+Tests use synthetic inputs (not cert-derived) so that orchestration
+behaviour is verified independently of the cert→inputs mapper (S-A7b).
+
+Reference: SAP 10.3 specification (13-01-2026) §§5-13 + Table 9c (the
+worksheet step list) + Table 12 (Energy Cost Deflator 0.36).
+"""
+
+from __future__ import annotations
+
+from dataclasses import replace
+
+import pytest
+
+from domain.sap.calculator import (
+    CalculatorInputs,
+    SapResult,
+    WindowInput,
+    calculate_sap_from_inputs,
+)
+from domain.sap.worksheet.dimensions import Dimensions
+from domain.sap.worksheet.heat_transmission import HeatTransmission
+from domain.sap.worksheet.solar_gains import Orientation
+
+
+def _baseline_inputs() -> CalculatorInputs:
+    """Reference dwelling for orchestrator tests — a 100 m² semi-detached
+    gas-boiler home in UK-average climate. Numbers chosen to land roughly
+    where a real RdSAP cert would: HLC ~150 W/K, τ ~100 h, SAP ~70."""
+    dim = Dimensions(
+        total_floor_area_m2=100.0,
+        volume_m3=250.0,
+        storey_count=2,
+        avg_storey_height_m=2.5,
+        ground_floor_area_m2=50.0,
+        ground_floor_perimeter_m=30.0,
+        top_floor_area_m2=50.0,
+        gross_wall_area_m2=150.0,
+        party_wall_area_m2=50.0,
+    )
+    ht = HeatTransmission(
+        walls_w_per_k=60.0,
+        roof_w_per_k=20.0,
+        floor_w_per_k=20.0,
+        party_walls_w_per_k=0.0,
+        windows_w_per_k=25.0,
+        doors_w_per_k=5.0,
+        thermal_bridging_w_per_k=20.0,
+        total_w_per_k=150.0,
+    )
+    windows = (
+        WindowInput(
+            area_m2=4.0,
+            orientation=Orientation.S,
+            pitch_deg=90.0,
+            g_perpendicular=0.63,
+            frame_factor=0.7,
+            overshading_factor=0.77,
+        ),
+        WindowInput(
+            area_m2=4.0,
+            orientation=Orientation.N,
+            pitch_deg=90.0,
+            g_perpendicular=0.63,
+            frame_factor=0.7,
+            overshading_factor=0.77,
+        ),
+    )
+    return CalculatorInputs(
+        dimensions=dim,
+        heat_transmission=ht,
+        infiltration_ach=0.7,
+        region=0,
+        windows=windows,
+        control_type=2,
+        responsiveness=1.0,
+        living_area_fraction=0.30,
+        control_temperature_adjustment_c=0.0,
+        thermal_mass_parameter_kj_per_m2_k=250.0,
+        main_heating_efficiency=0.85,
+        hot_water_kwh_per_yr=2400.0,
+        pumps_fans_kwh_per_yr=100.0,
+        lighting_kwh_per_yr=600.0,
+        fuel_unit_cost_gbp_per_kwh=0.07,
+        co2_factor_kg_per_kwh=0.21,
+    )
+
+
+def test_calculator_returns_twelve_month_breakdown_and_plausible_sap_score() -> None:
+    # Arrange — baseline 100 m² gas-boiler dwelling in UK-average climate.
+    inputs = _baseline_inputs()
+
+    # Act
+    result = calculate_sap_from_inputs(inputs)
+
+    # Assert — sanity, not exact: tracer bullet that the 12-month loop runs
+    # end-to-end and lands in a believable SAP band for the inputs.
+    assert isinstance(result, SapResult)
+    assert len(result.monthly) == 12
+    assert 1 <= result.sap_score <= 100
+    assert result.space_heating_kwh_per_yr > 0
+    assert result.total_fuel_cost_gbp > 0
+    assert result.ecf > 0
+    # The "main_heating_fuel + hot_water + pumps_fans + lighting" totals
+    # must reconcile with the cost line through the fuel unit cost.
+    expected_fuel = (
+        result.main_heating_fuel_kwh_per_yr
+        + result.hot_water_kwh_per_yr
+        + result.pumps_fans_kwh_per_yr
+        + result.lighting_kwh_per_yr
+    )
+    assert result.total_fuel_cost_gbp == pytest.approx(
+        expected_fuel * inputs.fuel_unit_cost_gbp_per_kwh, rel=1e-6
+    )
+
+
+def test_higher_main_heating_efficiency_reduces_fuel_use() -> None:
+    # Arrange — Direction check: doubling the boiler efficiency must halve
+    # the main-heating fuel kWh, holding everything else constant.
+    base = _baseline_inputs()
+    high_eff = replace(base, main_heating_efficiency=base.main_heating_efficiency * 2.0)
+
+    # Act
+    r_base = calculate_sap_from_inputs(base)
+    r_high = calculate_sap_from_inputs(high_eff)
+
+    # Assert
+    assert r_base.space_heating_kwh_per_yr == pytest.approx(
+        r_high.space_heating_kwh_per_yr, rel=1e-6
+    )
+    assert r_high.main_heating_fuel_kwh_per_yr == pytest.approx(
+        r_base.main_heating_fuel_kwh_per_yr / 2.0, rel=1e-6
+    )
+    assert r_high.sap_score >= r_base.sap_score
+
+
+def test_colder_climate_region_increases_space_heating_demand() -> None:
+    # Arrange — Direction check: same dwelling in Shetland (region 20) must
+    # require more space-heating kWh than in Thames (region 1) because the
+    # external-temperature column in Table U1 is consistently lower.
+    base = _baseline_inputs()
+    thames = replace(base, region=1)
+    shetland = replace(base, region=20)
+
+    # Act
+    r_thames = calculate_sap_from_inputs(thames)
+    r_shetland = calculate_sap_from_inputs(shetland)
+
+    # Assert
+    assert r_shetland.space_heating_kwh_per_yr > r_thames.space_heating_kwh_per_yr
+
+
+def test_zero_heat_transmission_collapses_space_heating_to_zero() -> None:
+    # Arrange — When HLC = 0 (perfect envelope) and there's no ventilation
+    # heat loss, no month can have a positive loss rate, so space heating
+    # must be zero across the year. Demonstrates the η-clamp in the loss
+    # path doesn't introduce spurious demand.
+    base = _baseline_inputs()
+    no_loss = replace(
+        base,
+        heat_transmission=HeatTransmission(0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0),
+        infiltration_ach=0.0,
+    )
+
+    # Act
+    result = calculate_sap_from_inputs(no_loss)
+
+    # Assert
+    assert result.space_heating_kwh_per_yr == 0.0
+    assert result.main_heating_fuel_kwh_per_yr == 0.0
+
+
+def test_ecf_uses_table_12_energy_cost_deflator() -> None:
+    # Arrange — §13 Equation (7): ECF = 0.36 × cost / (TFA + 45). The
+    # orchestrator must report an ECF that reconciles with this formula
+    # given the cost it reported.
+    inputs = _baseline_inputs()
+
+    # Act
+    result = calculate_sap_from_inputs(inputs)
+
+    # Assert
+    expected_ecf = (
+        0.36
+        * result.total_fuel_cost_gbp
+        / (inputs.dimensions.total_floor_area_m2 + 45.0)
+    )
+    assert result.ecf == pytest.approx(expected_ecf, rel=1e-6)