Calvert Responsible Index Series N-CSRSCertified semi-annual shareholder report (management investment company)

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As a Fund shareholder, you incur two types of costs: (1) transaction costs, including sales charges (loads) on purchases and redemption fees (if applicable); and (2) ongoing costs, including management fees; distribution and/or service fees; and other Fund expenses. This Example is intended to help you understand your ongoing costs (in dollars) of Fund investing and to compare these costs with the ongoing costs of investing in other mutual funds. The Example is based on an investment of $1,000 invested at the beginning of the period and held for the entire period (October 1, 2020 to March 31, 2021).

The first section of the table below provides information about actual account values and actual expenses. You may use the information in this section, together with the amount you invested, to estimate the expenses that you paid over the period. Simply divide your account value by $1,000 (for example, an $8,600 account value divided by $1,000 = 8.6), then multiply the result by the number in the first section under the heading entitled “Expenses Paid During Period” to estimate the expenses you paid on your account during this period.

The second section of the table below provides information about hypothetical account values and hypothetical expenses based on the actual Fund expense ratio and an assumed rate of return of 5% per year (before expenses), which is not the actual Fund return. The hypothetical account values and expenses may not be used to estimate the actual ending account balance or expenses you paid for the period. You may use this information to compare the ongoing costs of investing in the Fund and other funds. To do so, compare this 5% hypothetical example with the 5% hypothetical examples that appear in the shareholder reports of the other funds.

Please note that the expenses shown in the table are meant to highlight your ongoing costs only and do not reflect any transactional costs, such as sales charges (loads) or redemption fees (if applicable). Therefore, the second section of the table is useful in comparing ongoing costs only, and will not help you determine the relative total costs of owning different funds. In addition, if these transactional costs were included, your costs would be higher.

Calvert US Large-Cap Core Responsible Index Fund (the Fund) is a diversified series of Calvert Responsible Index Series, Inc. (the Corporation). The Corporation is a Maryland corporation registered under the Investment Company Act of 1940, as amended (the 1940 Act), as an open-end management investment company. The investment objective of the Fund is to seek to track the performance of the Calvert US Large-Cap Core Responsible Index, which measures the investment return of large-capitalization stocks.

The Fund offers four classes of shares. Class A shares are generally sold subject to a sales charge imposed at time of purchase. A contingent deferred sales charge of 0.25% may apply to certain redemptions of Class A shares for accounts for which no sales charge was paid, if redeemed within 12 months of purchase. Class C shares are sold without a front-end sales charge, and with certain exceptions, are charged a contingent deferred sales charge of 1% on shares redeemed within 12 months of purchase. Class C shares are only available for purchase through a financial intermediary. Effective January 25, 2019, Class C shares generally automatically convert to Class A shares ten years after their purchase and, effective November 5, 2020, automatically convert to Class A shares eight years after their purchase as described in the Fund's prospectus. Class I and Class R6 shares are sold at net asset value, are not subject to a sales charge and are sold only to certain eligible investors. Each class represents a pro rata interest in the Fund, but votes separately on class-specific matters and is subject to different expenses.

The Fund applies the accounting and reporting guidance in the Financial Accounting Standards Board (FASB) Accounting Standards Codification Topic 946, Financial Services – Investment Companies (ASC 946). Subsequent events, if any, through the date that the financial statements were issued have been evaluated in the preparation of the financial statements.

U.S. generally accepted accounting principles (U.S. GAAP) establishes a disclosure hierarchy that categorizes the inputs to valuation techniques used to value assets and liabilities at measurement date. These inputs are summarized in the three broad levels listed below:

Level 1 - quoted prices in active markets for identical securities

Level 2 - other significant observable inputs (including quoted prices for similar securities, interest rates, prepayment speeds, credit risk, etc.)

Level 3 - significant unobservable inputs (including the Fund’s own assumptions in determining the fair value of investments)

The inputs or methodologies used for valuing securities are not necessarily an indication of the risk associated with investing in those securities.

Valuation techniques used to value the Fund’s investments by major category are as follows:

The values assigned to fair value investments are based on available information and do not necessarily represent amounts that might ultimately be realized. Further, due to the inherent uncertainty of valuations of such investments, the fair values may differ significantly from the values that would have been used had an active market existed, and the differences could be material.

The following table summarizes the market value of the Fund's holdings as of March 31, 2021, based on the inputs used to value them:

Management has analyzed the Fund's tax positions taken for all open federal income tax years and has concluded that no provision for federal income tax is required in the Fund's financial statements. A Fund's federal tax return is subject to examination by the Internal Revenue Service for a period of three years from the date of filing.

The investment advisory fee is earned by CRM as compensation for investment advisory services rendered to the Fund. On March 1, 2021, Morgan Stanley acquired Eaton Vance Corp. (the “Transaction”) and CRM became an indirect, wholly-owned subsidiary of Morgan Stanley. In connection with the Transaction, the Fund entered into a new investment advisory agreement (the “New Agreement”) with CRM, which took effect on March 1, 2021. Pursuant to the New Agreement (and the Fund’s investment advisory agreement with CRM in effect prior to March 1, 2021), the fee is computed at the annual rate of 0.12% of the Fund's average daily net assets and is payable monthly. For the six months ended March 31, 2021, the investment advisory fee amounted to $2,104,159. The Fund may invest its cash in Cash Reserves Fund. CRM does not currently receive a fee for advisory services provided to Cash Reserves Fund.

CRM has agreed to reimburse the Fund’s operating expenses to the extent that total annual operating expenses (relating to ordinary operating expenses only and excluding expenses such as brokerage commissions, acquired fund fees and expenses of unaffiliated funds, borrowing costs, taxes or litigation expenses) exceed 0.49%, 1.24%, 0.24% and 0.19% for Class A, Class C, Class I and Class R6, respectively, of such class’s average daily net assets. The expense reimbursement agreement with CRM may be changed or terminated after January 31, 2022. For the six months ended March 31, 2021, CRM waived or reimbursed expenses of $1,790,660.

The administrative fee is earned by CRM as compensation for administrative services rendered to the Fund. The fee is computed at an annual rate of 0.12% of the Fund’s average daily net assets attributable to Class A, Class C, Class I and Class R6 and is payable monthly. For the six months ended March 31, 2021, CRM was paid administrative fees of $2,104,159.

The Fund has in effect a distribution plan for Class A shares (Class A Plan) pursuant to Rule 12b-1 under the 1940 Act. Pursuant to the Class A Plan, the Fund pays Eaton Vance Distributors, Inc. (EVD), an affiliate of CRM and the Fund’s principal underwriter, a distribution and service fee of 0.25% per annum of its average daily net assets attributable to Class A shares for distribution services and facilities provided to the Fund by EVD, as well as for personal services and/or the maintenance of shareholder accounts. The Fund also has in effect a distribution plan for Class C shares (Class C Plan) pursuant to Rule 12b-1 under the 1940 Act. Pursuant to the Class C Plan, the Fund pays EVD amounts equal to 0.75% per annum of its average daily net assets attributable to Class C shares for providing ongoing distribution services and facilities to the Fund. In addition, pursuant to the Class C Plan, the Fund also makes payments of service fees to EVD, financial intermediaries and other persons in amounts equal to 0.25% per annum of its average daily net assets attributable to that class. Service fees paid or accrued are for personal services and/or the maintenance of shareholder accounts. Distribution and service fees paid or accrued for the six months ended March 31, 2021 amounted to $630,872 and $334,708 for Class A shares and Class C shares, respectively.

The Fund was informed that EVD received $72,445 as its portion of the sales charge on sales of Class A shares for the six months ended March 31, 2021. The Fund was also informed that EVD received $111 and $1,489 of contingent deferred sales charges paid by Class A and Class C shareholders, respectively, for the same period.

Eaton Vance Management (EVM), an affiliate of CRM, provides sub-transfer agency and related services to the Fund pursuant to a Sub-Transfer Agency Support Services Agreement. For the six months ended March 31, 2021, sub-transfer agency fees and expenses incurred to EVM amounted to $62,341 and are included in transfer agency fees and expenses on the Statement of Operations.

Each Director of the Fund who is not an employee of CRM or its affiliates receives an annual fee of $154,000, plus an annual Committee fee ranging from $8,500 to $16,500 depending on the Committee. The Board chair receives an additional $30,000 ($20,000 prior to January 1, 2021) annual fee and Committee chairs receive an additional $6,000 annual fee. Eligible Directors may participate in a Deferred Compensation Plan (the Plan). Amounts deferred under the Plan are treated as though equal dollar amounts had been invested in shares of the Fund or other Calvert funds selected by the Directors. The Fund purchases shares of the funds selected equal to the dollar amounts deferred under the Plan, resulting in an asset equal to the deferred compensation liability. Obligations of the Plan are paid solely from the Fund's assets. Directors’ fees are allocated to each of the Calvert funds served. Salaries and fees of officers and Directors of the Fund who are employees of CRM or its affiliates are paid by CRM. Prior to December 31, 2020, an Advisory Council aided the Board and CRM in advancing the cause of responsible investing through original scholarship and thought leadership. The Advisory Council consisted of CRM’s Chief Executive Officer and three additional members. For the year ended December 31, 2020, each member (other than CRM’s Chief Executive Officer) was compensated $20,000 for their service on the Advisory Council. Such compensation, and any other compensation and/or expenses incurred by the Advisory Council as may be approved by the Board, was borne by the Calvert funds. For the six months ended March 31, 2021, the Fund’s allocated portion of the Advisory Council compensation and fees was $3,576, which is included in miscellaneous expense on the Statement of Operations.

During the six months ended March 31, 2021, the cost of purchases and proceeds from sales of investments, other than short-term securities, were $679,796,339 and $155,512,286, respectively.

At September 30, 2020, the Fund, for federal income tax purposes, had deferred capital losses of $14,515,504 which would reduce the Fund's taxable income arising from future net realized gains on investment transactions, if any, to the extent permitted by the Internal Revenue Code, and thus would reduce the amount of distributions to shareholders, which would otherwise be necessary to relieve the Fund of any liability for federal income or excise tax. The deferred capital losses are treated as arising on the first day of the Fund's next taxable year, can be carried forward for an unlimited period, and retain the same short-term or long-term character as when originally deferred. Of the deferred capital losses at September 30, 2020, $9,873,286 are short-term and $4,642,218 are long-term.

The cost and unrealized appreciation (depreciation) of investments of the Fund at March 31, 2021, as determined on a federal income tax basis, were as follows: