Summary of significant accounting policies (Policies)	3 Months Ended
Summary of significant accounting policies (Policies)	Mar. 31, 2020
Summary of significant accounting policies
Use of estimates	Use of estimates The preparation of the condensed consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses, revenue recognition, the fair value of ordinary shares and the valuation of the warrant liability prior to the Company’s IPO, share-based compensation expense, and income taxes. The Company bases its estimates on historical experience, known trends and other market‑specific or other relevant factors that it believes to be reasonable under the circumstances. Estimates are periodically reviewed in light of reasonable changes in circumstances, facts and experience and may be changed as new events occur and additional information is obtained. As of the date of issuance of these financial statements, the Company is not aware of any specific event or circumstance that would require the Company to update its estimates, assumptions and judgments or revise the carrying value of its assets or liabilities. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates or assumptions, and any such differences may be material to the Company’s financial statements.
Significant Risks and Uncertainties	Significant Risks and Uncertainties With the global spread of the ongoing COVID-19 pandemic in the first quarter of 2020, the Company established a cross-functional task force and implemented business continuity plans designed to address and mitigate the impact of the COVID-19 pandemic on the Company’s business. While the Company is experiencing limited financial impacts at this time, the extent to which the COVID-19 pandemic impacts the Company’s business, clinical development and regulatory efforts, corporate development objectives and the value of and market for the Company’s ADSs, will depend on future developments that are highly uncertain and cannot be predicted with confidence at this time, such as the ultimate duration of the pandemic, travel restrictions, quarantines, social distancing and business closure requirements in the United Kingdom, United States, Europe and other countries, and the effectiveness of actions taken globally to contain and treat the disease. The global economic slowdown, the overall disruption of global healthcare systems and the other risks and uncertainties associated with the pandemic could have a material adverse effect on the Company’s business, financial condition, results of operations and growth prospects. In addition, the Company is subject to other challenges and risks specific to its business and ability to execute its strategy, as well as risks and uncertainties common to companies in the biotechnology industry, including but not limited to: risks of failure of preclinical studies and clinical trials, the need to obtain marketing approval for any drug product candidate that it may identify and develop, the need to successfully commercialize and gain market acceptance of its product candidates, dependence on key personnel and collaboration partners, protection of proprietary technology, compliance with government regulations, development by competitors of technological innovations, and the ability to secure additional capital to fund operations. In addition, to the extent the ongoing COVID-19 pandemic adversely affects the Company’s business and results of operations, it may also have the effect of heightening many of the other risks and uncertainties discussed above.
Unaudited Interim Financial Information	Unaudited Interim Financial Information Certain information in the footnote disclosures of the financial statements has been condensed or omitted pursuant to the rules and regulations of the SEC. These unaudited condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and notes thereto for the year ended December 31, 2019 included in the Company’s 2019 Annual Report. The accompanying condensed consolidated balance sheet at March 31, 2020, condensed consolidated statements of operations and comprehensive loss, condensed consolidated statements of convertible preferred shares and shareholders’ equity (deficit) and condensed consolidated statements of cash flows for the three months ended March 31, 2020 and 2019, and the related financial information disclosed in these notes are unaudited. The unaudited interim financial statements have been prepared on the same basis as the audited annual financial statements for the year ended December 31, 2019, and, in the opinion of management, reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of March 31, 2020, the results of its operations for the three months ended March 31, 2020 and 2019, and its cash flows for the three months ended March 31, 2020 and 2019. The results for the three months ended March 31, 2020 are not necessarily indicative of the results to be expected for the year ending December 31, 2020, any other interim periods, or any future year or period.
Foreign currency and currency translation	Foreign currency and currency translation The functional currency is the currency of the primary economic environment in which an entity’s operations are conducted. On June 1, 2019, Bicycle Therapeutics plc adopted the U.S. dollar (“USD”) as its functional currency. Bicycle Therapeutics plc is a holding company that has no operating activities and its primary functions are to serve as a financing vehicle to fund the operations of the Company’s operating entities, to serve as the listing company needed to access U.S. capital markets, and to hold investments. Therefore, its financing source is the primary indicator of its cash flows and its functional currency. The change in functional currency from the British Pound Sterling is due to a change in the source of Bicycle Therapeutics plc’s financing and cash flows, which following the completion of the IPO is now primarily the U.S. dollar. Historically its financing had been in British Pound Sterling. The functional currency of Bicycle Therapeutics plc’s wholly owned non-U.S. subsidiaries, BicycleTx Limited and BicycleRD Limited, is the British Pound Sterling and the functional currency of its U.S. subsidiary, Bicycle Therapeutics Inc. is the USD. The functional currency of the Company’s subsidiaries is the same as the local currency. Monetary assets and liabilities denominated in currencies other than the functional currency are remeasured into the functional currency at rates of exchange prevailing at the balance sheet dates. Non-monetary assets and liabilities denominated in foreign currencies are remeasured into the functional currency at the exchange rates prevailing at the date of the transaction. Exchange gains or losses arising from foreign currency transactions are included in the determination of net loss for the respective periods. Adjustments that arise from exchange rate changes on transactions denominated in a currency other than the local currency are included in general and administrative expense in the condensed consolidated statements of operations and comprehensive loss as incurred. The Company recorded a foreign exchange gain of $1.3 million and an exchange loss of $0.3 million during the three months ended March 31, 2020 and 2019, respectively. The Company translates the assets and liabilities of BicycleTx Limited and BicycleRD Limited into USD at the exchange rate in effect on the balance sheet date. Revenues and expenses are translated at the average exchange rate in effect during the period. Unrealized translation gains and losses are recorded as a foreign currency translation adjustment, which is included in the condensed consolidated statements of convertible preferred shares and shareholders’ equity (deficit) as a component of accumulated other comprehensive income (loss).
Recently issued accounting pronouncements	Recently issued accounting pronouncements In June 2016, the FASB issued ASU No. 2016-13, Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”). ASU 2016-13 will change how companies account for credit losses for most financial assets and certain other instruments. For trade receivables, loans and held-to-maturity debt securities, companies will be required to recognize an allowance for credit losses rather than reducing the carrying value of the asset. ASU 2016-13 was effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. In November 2019, the FASB issued ASU No. 2019-10, Financial Instruments — Credit Losses (Topic 326), Derivatives and Hedging (Topic 815), and Leases (Topic 842): Effective Dates to amend the effective date of ASU 2016-13, for entities eligible to be smaller reporting companies as defined by the SEC, to be effective for fiscal years beginning after December 15, 2022, including interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the potential impact that the adoption of ASU 2016-13 will have on the Company’s financial position and results of operations